20 files changed, 1002 insertions, 271 deletions

diff --git a/Makefile b/Makefile
index 8d9f11e75e..a5b6eebf1a 100644
--- a/Makefile
+++ b/Makefile
@@ -226,7 +226,7 @@ BASIC_CFLAGS =
 BASIC_LDFLAGS =
 
 SCRIPT_SH = \
-	git-bisect.sh git-checkout.sh \
+	git-bisect.sh \
 	git-clone.sh \
 	git-merge-one-file.sh git-mergetool.sh git-parse-remote.sh \
 	git-pull.sh git-rebase.sh git-rebase--interactive.sh \
@@ -265,23 +265,23 @@ PROGRAMS = \
 	git-upload-pack$X \
 	git-pack-redundant$X git-var$X \
 	git-merge-tree$X git-imap-send$X \
-	git-merge-recursive$X \
 	$(EXTRA_PROGRAMS)
 
 # Empty...
 EXTRA_PROGRAMS =
 
+# List built-in command $C whose implementation cmd_$C() is not in
+# builtin-$C.o but is linked in as part of some other command.
 BUILT_INS = \
 	git-format-patch$X git-show$X git-whatchanged$X git-cherry$X \
 	git-get-tar-commit-id$X git-init$X git-repo-config$X \
 	git-fsck-objects$X git-cherry-pick$X git-peek-remote$X git-status$X \
+	git-merge-subtree$X \
 	$(patsubst builtin-%.o,git-%$X,$(BUILTIN_OBJS))
 
 # what 'all' will build and 'install' will install, in gitexecdir
 ALL_PROGRAMS = $(PROGRAMS) $(SCRIPTS)
 
-ALL_PROGRAMS += git-merge-subtree$X
-
 # what 'all' will build but not install in gitexecdir
 OTHER_PROGRAMS = git$X gitweb/gitweb.cgi
 
@@ -327,7 +327,7 @@ LIB_OBJS = \
 	alloc.o merge-file.o path-list.o help.o unpack-trees.o $(DIFF_OBJS) \
 	color.o wt-status.o archive-zip.o archive-tar.o shallow.o utf8.o \
 	convert.o attr.o decorate.o progress.o mailmap.o symlinks.o remote.o \
-	transport.o bundle.o walker.o parse-options.o ws.o archive.o \
+	transport.o bundle.o walker.o parse-options.o ws.o archive.o branch.o \
 	alias.o
 
 BUILTIN_OBJS = \
@@ -340,6 +340,7 @@ BUILTIN_OBJS = \
 	builtin-bundle.o \
 	builtin-cat-file.o \
 	builtin-check-attr.o \
+	builtin-checkout.o \
 	builtin-checkout-index.o \
 	builtin-check-ref-format.o \
 	builtin-clean.o \
@@ -370,6 +371,7 @@ BUILTIN_OBJS = \
 	builtin-merge-base.o \
 	builtin-merge-file.o \
 	builtin-merge-ours.o \
+	builtin-merge-recursive.o \
 	builtin-mv.o \
 	builtin-name-rev.o \
 	builtin-pack-objects.o \
@@ -840,9 +842,6 @@ help.o: help.c common-cmds.h GIT-CFLAGS
 		'-DGIT_MAN_PATH="$(mandir_SQ)"' \
 		'-DGIT_INFO_PATH="$(infodir_SQ)"' $<
 
-git-merge-subtree$X: git-merge-recursive$X
-	$(QUIET_BUILT_IN)$(RM) $@ && ln git-merge-recursive$X $@
-
 $(BUILT_INS): git$X
 	$(QUIET_BUILT_IN)$(RM) $@ && ln git$X $@
 
diff --git a/branch.c b/branch.c
new file mode 100644
index 0000000000..1fc8788897
--- /dev/null
+++ b/branch.c
@@ -0,0 +1,148 @@
+#include "cache.h"
+#include "branch.h"
+#include "refs.h"
+#include "remote.h"
+#include "commit.h"
+
+struct tracking {
+	struct refspec spec;
+	char *src;
+	const char *remote;
+	int matches;
+};
+
+static int find_tracked_branch(struct remote *remote, void *priv)
+{
+	struct tracking *tracking = priv;
+
+	if (!remote_find_tracking(remote, &tracking->spec)) {
+		if (++tracking->matches == 1) {
+			tracking->src = tracking->spec.src;
+			tracking->remote = remote->name;
+		} else {
+			free(tracking->spec.src);
+			if (tracking->src) {
+				free(tracking->src);
+				tracking->src = NULL;
+			}
+		}
+		tracking->spec.src = NULL;
+	}
+
+	return 0;
+}
+
+/*
+ * This is called when new_ref is branched off of orig_ref, and tries
+ * to infer the settings for branch.<new_ref>.{remote,merge} from the
+ * config.
+ */
+static int setup_tracking(const char *new_ref, const char *orig_ref)
+{
+	char key[1024];
+	struct tracking tracking;
+
+	if (strlen(new_ref) > 1024 - 7 - 7 - 1)
+		return error("Tracking not set up: name too long: %s",
+				new_ref);
+
+	memset(&tracking, 0, sizeof(tracking));
+	tracking.spec.dst = (char *)orig_ref;
+	if (for_each_remote(find_tracked_branch, &tracking) ||
+			!tracking.matches)
+		return 1;
+
+	if (tracking.matches > 1)
+		return error("Not tracking: ambiguous information for ref %s",
+				orig_ref);
+
+	if (tracking.matches == 1) {
+		sprintf(key, "branch.%s.remote", new_ref);
+		git_config_set(key, tracking.remote ?  tracking.remote : ".");
+		sprintf(key, "branch.%s.merge", new_ref);
+		git_config_set(key, tracking.src);
+		free(tracking.src);
+		printf("Branch %s set up to track remote branch %s.\n",
+			       new_ref, orig_ref);
+	}
+
+	return 0;
+}
+
+void create_branch(const char *head,
+		   const char *name, const char *start_name,
+		   int force, int reflog, int track)
+{
+	struct ref_lock *lock;
+	struct commit *commit;
+	unsigned char sha1[20];
+	char *real_ref, ref[PATH_MAX], msg[PATH_MAX + 20];
+	int forcing = 0;
+
+	snprintf(ref, sizeof ref, "refs/heads/%s", name);
+	if (check_ref_format(ref))
+		die("'%s' is not a valid branch name.", name);
+
+	if (resolve_ref(ref, sha1, 1, NULL)) {
+		if (!force)
+			die("A branch named '%s' already exists.", name);
+		else if (!is_bare_repository() && !strcmp(head, name))
+			die("Cannot force update the current branch.");
+		forcing = 1;
+	}
+
+	real_ref = NULL;
+	if (get_sha1(start_name, sha1))
+		die("Not a valid object name: '%s'.", start_name);
+
+	switch (dwim_ref(start_name, strlen(start_name), sha1, &real_ref)) {
+	case 0:
+		/* Not branching from any existing branch */
+		real_ref = NULL;
+		break;
+	case 1:
+		/* Unique completion -- good */
+		break;
+	default:
+		die("Ambiguous object name: '%s'.", start_name);
+		break;
+	}
+
+	if ((commit = lookup_commit_reference(sha1)) == NULL)
+		die("Not a valid branch point: '%s'.", start_name);
+	hashcpy(sha1, commit->object.sha1);
+
+	lock = lock_any_ref_for_update(ref, NULL, 0);
+	if (!lock)
+		die("Failed to lock ref for update: %s.", strerror(errno));
+
+	if (reflog)
+		log_all_ref_updates = 1;
+
+	if (forcing)
+		snprintf(msg, sizeof msg, "branch: Reset from %s",
+			 start_name);
+	else
+		snprintf(msg, sizeof msg, "branch: Created from %s",
+			 start_name);
+
+	/* When branching off a remote branch, set up so that git-pull
+	   automatically merges from there.  So far, this is only done for
+	   remotes registered via .git/config.  */
+	if (real_ref && track)
+		setup_tracking(name, real_ref);
+
+	if (write_ref_sha1(lock, sha1, msg) < 0)
+		die("Failed to write ref: %s.", strerror(errno));
+
+	if (real_ref)
+		free(real_ref);
+}
+
+void remove_branch_state(void)
+{
+	unlink(git_path("MERGE_HEAD"));
+	unlink(git_path("rr-cache/MERGE_RR"));
+	unlink(git_path("MERGE_MSG"));
+	unlink(git_path("SQUASH_MSG"));
+}
diff --git a/branch.h b/branch.h
new file mode 100644
index 0000000000..d30abe0369
--- /dev/null
+++ b/branch.h
@@ -0,0 +1,24 @@
+#ifndef BRANCH_H
+#define BRANCH_H
+
+/* Functions for acting on the information about branches. */
+
+/*
+ * Creates a new branch, where head is the branch currently checked
+ * out, name is the new branch name, start_name is the name of the
+ * existing branch that the new branch should start from, force
+ * enables overwriting an existing (non-head) branch, reflog creates a
+ * reflog for the branch, and track causes the new branch to be
+ * configured to merge the remote branch that start_name is a tracking
+ * branch for (if any).
+ */
+void create_branch(const char *head, const char *name, const char *start_name,
+		   int force, int reflog, int track);
+
+/*
+ * Remove information about the state of working on the current
+ * branch. (E.g., MERGE_HEAD)
+ */
+void remove_branch_state(void);
+
+#endif
diff --git a/builtin-branch.c b/builtin-branch.c
index 9edf2eb816..7e991030ca 100644
--- a/builtin-branch.c
+++ b/builtin-branch.c
@@ -12,6 +12,7 @@
 #include "builtin.h"
 #include "remote.h"
 #include "parse-options.h"
+#include "branch.h"
 
 static const char * const builtin_branch_usage[] = {
 	"git-branch [options] [-r | -a]",
@@ -359,141 +360,6 @@ static void print_ref_list(int kinds, int detached, int verbose, int abbrev, str
 	free_ref_list(&ref_list);
 }
 
-struct tracking {
-	struct refspec spec;
-	char *src;
-	const char *remote;
-	int matches;
-};
-
-static int find_tracked_branch(struct remote *remote, void *priv)
-{
-	struct tracking *tracking = priv;
-
-	if (!remote_find_tracking(remote, &tracking->spec)) {
-		if (++tracking->matches == 1) {
-			tracking->src = tracking->spec.src;
-			tracking->remote = remote->name;
-		} else {
-			free(tracking->spec.src);
-			if (tracking->src) {
-				free(tracking->src);
-				tracking->src = NULL;
-			}
-		}
-		tracking->spec.src = NULL;
-	}
-
-	return 0;
-}
-
-
-/*
- * This is called when new_ref is branched off of orig_ref, and tries
- * to infer the settings for branch.<new_ref>.{remote,merge} from the
- * config.
- */
-static int setup_tracking(const char *new_ref, const char *orig_ref)
-{
-	char key[1024];
-	struct tracking tracking;
-
-	if (strlen(new_ref) > 1024 - 7 - 7 - 1)
-		return error("Tracking not set up: name too long: %s",
-				new_ref);
-
-	memset(&tracking, 0, sizeof(tracking));
-	tracking.spec.dst = (char *)orig_ref;
-	if (for_each_remote(find_tracked_branch, &tracking) ||
-			!tracking.matches)
-		return 1;
-
-	if (tracking.matches > 1)
-		return error("Not tracking: ambiguous information for ref %s",
-				orig_ref);
-
-	if (tracking.matches == 1) {
-		sprintf(key, "branch.%s.remote", new_ref);
-		git_config_set(key, tracking.remote ?  tracking.remote : ".");
-		sprintf(key, "branch.%s.merge", new_ref);
-		git_config_set(key, tracking.src);
-		free(tracking.src);
-		printf("Branch %s set up to track remote branch %s.\n",
-			       new_ref, orig_ref);
-	}
-
-	return 0;
-}
-
-static void create_branch(const char *name, const char *start_name,
-			  int force, int reflog, int track)
-{
-	struct ref_lock *lock;
-	struct commit *commit;
-	unsigned char sha1[20];
-	char *real_ref, ref[PATH_MAX], msg[PATH_MAX + 20];
-	int forcing = 0;
-
-	snprintf(ref, sizeof ref, "refs/heads/%s", name);
-	if (check_ref_format(ref))
-		die("'%s' is not a valid branch name.", name);
-
-	if (resolve_ref(ref, sha1, 1, NULL)) {
-		if (!force)
-			die("A branch named '%s' already exists.", name);
-		else if (!is_bare_repository() && !strcmp(head, name))
-			die("Cannot force update the current branch.");
-		forcing = 1;
-	}
-
-	real_ref = NULL;
-	if (get_sha1(start_name, sha1))
-		die("Not a valid object name: '%s'.", start_name);
-
-	switch (dwim_ref(start_name, strlen(start_name), sha1, &real_ref)) {
-	case 0:
-		/* Not branching from any existing branch */
-		real_ref = NULL;
-		break;
-	case 1:
-		/* Unique completion -- good */
-		break;
-	default:
-		die("Ambiguous object name: '%s'.", start_name);
-		break;
-	}
-
-	if ((commit = lookup_commit_reference(sha1)) == NULL)
-		die("Not a valid branch point: '%s'.", start_name);
-	hashcpy(sha1, commit->object.sha1);
-
-	lock = lock_any_ref_for_update(ref, NULL, 0);
-	if (!lock)
-		die("Failed to lock ref for update: %s.", strerror(errno));
-
-	if (reflog)
-		log_all_ref_updates = 1;
-
-	if (forcing)
-		snprintf(msg, sizeof msg, "branch: Reset from %s",
-			 start_name);
-	else
-		snprintf(msg, sizeof msg, "branch: Created from %s",
-			 start_name);
-
-	/* When branching off a remote branch, set up so that git-pull
-	   automatically merges from there.  So far, this is only done for
-	   remotes registered via .git/config.  */
-	if (real_ref && track)
-		setup_tracking(name, real_ref);
-
-	if (write_ref_sha1(lock, sha1, msg) < 0)
-		die("Failed to write ref: %s.", strerror(errno));
-
-	if (real_ref)
-		free(real_ref);
-}
-
 static void rename_branch(const char *oldname, const char *newname, int force)
 {
 	char oldref[PATH_MAX], newref[PATH_MAX], logmsg[PATH_MAX*2 + 100];
@@ -618,7 +484,7 @@ int cmd_branch(int argc, const char **argv, const char *prefix)
 	else if (rename && (argc == 2))
 		rename_branch(argv[0], argv[1], rename > 1);
 	else if (argc <= 2)
-		create_branch(argv[0], (argc == 2) ? argv[1] : head,
+		create_branch(head, argv[0], (argc == 2) ? argv[1] : head,
 			      force_create, reflog, track);
 	else
 		usage_with_options(builtin_branch_usage, options);
diff --git a/builtin-checkout.c b/builtin-checkout.c
new file mode 100644
index 0000000000..d5f093094c
--- /dev/null
+++ b/builtin-checkout.c
@@ -0,0 +1,573 @@
+#include "cache.h"
+#include "builtin.h"
+#include "parse-options.h"
+#include "refs.h"
+#include "commit.h"
+#include "tree.h"
+#include "tree-walk.h"
+#include "unpack-trees.h"
+#include "dir.h"
+#include "run-command.h"
+#include "merge-recursive.h"
+#include "branch.h"
+#include "diff.h"
+#include "revision.h"
+#include "remote.h"
+
+static const char * const checkout_usage[] = {
+	"git checkout [options] <branch>",
+	"git checkout [options] [<branch>] -- <file>...",
+	NULL,
+};
+
+static int post_checkout_hook(struct commit *old, struct commit *new,
+			      int changed)
+{
+	struct child_process proc;
+	const char *name = git_path("hooks/post-checkout");
+	const char *argv[5];
+
+	if (access(name, X_OK) < 0)
+		return 0;
+
+	memset(&proc, 0, sizeof(proc));
+	argv[0] = name;
+	argv[1] = xstrdup(sha1_to_hex(old->object.sha1));
+	argv[2] = xstrdup(sha1_to_hex(new->object.sha1));
+	argv[3] = changed ? "1" : "0";
+	argv[4] = NULL;
+	proc.argv = argv;
+	proc.no_stdin = 1;
+	proc.stdout_to_stderr = 1;
+	return run_command(&proc);
+}
+
+static int update_some(const unsigned char *sha1, const char *base, int baselen,
+		       const char *pathname, unsigned mode, int stage)
+{
+	int len;
+	struct cache_entry *ce;
+
+	if (S_ISGITLINK(mode))
+		return 0;
+
+	if (S_ISDIR(mode))
+		return READ_TREE_RECURSIVE;
+
+	len = baselen + strlen(pathname);
+	ce = xcalloc(1, cache_entry_size(len));
+	hashcpy(ce->sha1, sha1);
+	memcpy(ce->name, base, baselen);
+	memcpy(ce->name + baselen, pathname, len - baselen);
+	ce->ce_flags = create_ce_flags(len, 0);
+	ce->ce_mode = create_ce_mode(mode);
+	add_cache_entry(ce, ADD_CACHE_OK_TO_ADD | ADD_CACHE_OK_TO_REPLACE);
+	return 0;
+}
+
+static int read_tree_some(struct tree *tree, const char **pathspec)
+{
+	int newfd;
+	struct lock_file *lock_file = xcalloc(1, sizeof(struct lock_file));
+	newfd = hold_locked_index(lock_file, 1);
+	read_cache();
+
+	read_tree_recursive(tree, "", 0, 0, pathspec, update_some);
+
+	if (write_cache(newfd, active_cache, active_nr) ||
+	    commit_locked_index(lock_file))
+		die("unable to write new index file");
+
+	/* update the index with the given tree's info
+	 * for all args, expanding wildcards, and exit
+	 * with any non-zero return code.
+	 */
+	return 0;
+}
+
+static int checkout_paths(const char **pathspec)
+{
+	int pos;
+	struct checkout state;
+	static char *ps_matched;
+	unsigned char rev[20];
+	int flag;
+	struct commit *head;
+
+	for (pos = 0; pathspec[pos]; pos++)
+		;
+	ps_matched = xcalloc(1, pos);
+
+	for (pos = 0; pos < active_nr; pos++) {
+		struct cache_entry *ce = active_cache[pos];
+		pathspec_match(pathspec, ps_matched, ce->name, 0);
+	}
+
+	if (report_path_error(ps_matched, pathspec, 0))
+		return 1;
+
+	memset(&state, 0, sizeof(state));
+	state.force = 1;
+	state.refresh_cache = 1;
+	for (pos = 0; pos < active_nr; pos++) {
+		struct cache_entry *ce = active_cache[pos];
+		if (pathspec_match(pathspec, NULL, ce->name, 0)) {
+			checkout_entry(ce, &state, NULL);
+		}
+	}
+
+	resolve_ref("HEAD", rev, 0, &flag);
+	head = lookup_commit_reference_gently(rev, 1);
+
+	return post_checkout_hook(head, head, 0);
+}
+
+static void show_local_changes(struct object *head)
+{
+	struct rev_info rev;
+	/* I think we want full paths, even if we're in a subdirectory. */
+	init_revisions(&rev, NULL);
+	rev.abbrev = 0;
+	rev.diffopt.output_format |= DIFF_FORMAT_NAME_STATUS;
+	add_pending_object(&rev, head, NULL);
+	run_diff_index(&rev, 0);
+}
+
+static void describe_detached_head(char *msg, struct commit *commit)
+{
+	struct strbuf sb;
+	strbuf_init(&sb, 0);
+	parse_commit(commit);
+	pretty_print_commit(CMIT_FMT_ONELINE, commit, &sb, 0, "", "", 0, 0);
+	fprintf(stderr, "%s %s... %s\n", msg,
+		find_unique_abbrev(commit->object.sha1, DEFAULT_ABBREV), sb.buf);
+	strbuf_release(&sb);
+}
+
+static int reset_to_new(struct tree *tree, int quiet)
+{
+	struct unpack_trees_options opts;
+	struct tree_desc tree_desc;
+	memset(&opts, 0, sizeof(opts));
+	opts.head_idx = -1;
+	opts.update = 1;
+	opts.reset = 1;
+	opts.merge = 1;
+	opts.fn = oneway_merge;
+	opts.verbose_update = !quiet;
+	parse_tree(tree);
+	init_tree_desc(&tree_desc, tree->buffer, tree->size);
+	if (unpack_trees(1, &tree_desc, &opts))
+		return 128;
+	return 0;
+}
+
+static void reset_clean_to_new(struct tree *tree, int quiet)
+{
+	struct unpack_trees_options opts;
+	struct tree_desc tree_desc;
+	memset(&opts, 0, sizeof(opts));
+	opts.head_idx = -1;
+	opts.skip_unmerged = 1;
+	opts.reset = 1;
+	opts.merge = 1;
+	opts.fn = oneway_merge;
+	opts.verbose_update = !quiet;
+	parse_tree(tree);
+	init_tree_desc(&tree_desc, tree->buffer, tree->size);
+	if (unpack_trees(1, &tree_desc, &opts))
+		exit(128);
+}
+
+struct checkout_opts {
+	int quiet;
+	int merge;
+	int force;
+
+	char *new_branch;
+	int new_branch_log;
+	int track;
+};
+
+struct branch_info {
+	const char *name; /* The short name used */
+	const char *path; /* The full name of a real branch */
+	struct commit *commit; /* The named commit */
+};
+
+static void setup_branch_path(struct branch_info *branch)
+{
+	struct strbuf buf;
+	strbuf_init(&buf, 0);
+	strbuf_addstr(&buf, "refs/heads/");
+	strbuf_addstr(&buf, branch->name);
+	branch->path = strbuf_detach(&buf, NULL);
+}
+
+static int merge_working_tree(struct checkout_opts *opts,
+			      struct branch_info *old, struct branch_info *new)
+{
+	int ret;
+	struct lock_file *lock_file = xcalloc(1, sizeof(struct lock_file));
+	int newfd = hold_locked_index(lock_file, 1);
+	read_cache();
+
+	if (opts->force) {
+		ret = reset_to_new(new->commit->tree, opts->quiet);
+		if (ret)
+			return ret;
+	} else {
+		struct tree_desc trees[2];
+		struct tree *tree;
+		struct unpack_trees_options topts;
+		memset(&topts, 0, sizeof(topts));
+		topts.head_idx = -1;
+
+		refresh_cache(REFRESH_QUIET);
+
+		if (unmerged_cache()) {
+			error("you need to resolve your current index first");
+			return 1;
+		}
+
+		/* 2-way merge to the new branch */
+		topts.update = 1;
+		topts.merge = 1;
+		topts.gently = opts->merge;
+		topts.verbose_update = !opts->quiet;
+		topts.fn = twoway_merge;
+		topts.dir = xcalloc(1, sizeof(*topts.dir));
+		topts.dir->show_ignored = 1;
+		topts.dir->exclude_per_dir = ".gitignore";
+		tree = parse_tree_indirect(old->commit->object.sha1);
+		init_tree_desc(&trees[0], tree->buffer, tree->size);
+		tree = parse_tree_indirect(new->commit->object.sha1);
+		init_tree_desc(&trees[1], tree->buffer, tree->size);
+
+		if (unpack_trees(2, trees, &topts)) {
+			/*
+			 * Unpack couldn't do a trivial merge; either
+			 * give up or do a real merge, depending on
+			 * whether the merge flag was used.
+			 */
+			struct tree *result;
+			struct tree *work;
+			if (!opts->merge)
+				return 1;
+			parse_commit(old->commit);
+
+			/* Do more real merge */
+
+			/*
+			 * We update the index fully, then write the
+			 * tree from the index, then merge the new
+			 * branch with the current tree, with the old
+			 * branch as the base. Then we reset the index
+			 * (but not the working tree) to the new
+			 * branch, leaving the working tree as the
+			 * merged version, but skipping unmerged
+			 * entries in the index.
+			 */
+
+			add_files_to_cache(0, NULL, NULL);
+			work = write_tree_from_memory();
+
+			ret = reset_to_new(new->commit->tree, opts->quiet);
+			if (ret)
+				return ret;
+			merge_trees(new->commit->tree, work, old->commit->tree,
+				    new->name, "local", &result);
+			reset_clean_to_new(new->commit->tree, opts->quiet);
+		}
+	}
+
+	if (write_cache(newfd, active_cache, active_nr) ||
+	    commit_locked_index(lock_file))
+		die("unable to write new index file");
+
+	if (!opts->force)
+		show_local_changes(&new->commit->object);
+
+	return 0;
+}
+
+static void report_tracking(struct branch_info *new, struct checkout_opts *opts)
+{
+	/*
+	 * We have switched to a new branch; is it building on
+	 * top of another branch, and if so does that other branch
+	 * have changes we do not have yet?
+	 */
+	char *base;
+	unsigned char sha1[20];
+	struct commit *ours, *theirs;
+	char symmetric[84];
+	struct rev_info revs;
+	const char *rev_argv[10];
+	int rev_argc;
+	int num_ours, num_theirs;
+	const char *remote_msg;
+	struct branch *branch = branch_get(new->name);
+
+	/*
+	 * Nothing to report unless we are marked to build on top of
+	 * somebody else.
+	 */
+	if (!branch || !branch->merge || !branch->merge[0] || !branch->merge[0]->dst)
+		return;
+
+	/*
+	 * If what we used to build on no longer exists, there is
+	 * nothing to report.
+	 */
+	base = branch->merge[0]->dst;
+	if (!resolve_ref(base, sha1, 1, NULL))
+		return;
+
+	theirs = lookup_commit(sha1);
+	ours = new->commit;
+	if (!hashcmp(sha1, ours->object.sha1))
+		return; /* we are the same */
+
+	/* Run "rev-list --left-right ours...theirs" internally... */
+	rev_argc = 0;
+	rev_argv[rev_argc++] = NULL;
+	rev_argv[rev_argc++] = "--left-right";
+	rev_argv[rev_argc++] = symmetric;
+	rev_argv[rev_argc++] = "--";
+	rev_argv[rev_argc] = NULL;
+
+	strcpy(symmetric, sha1_to_hex(ours->object.sha1));
+	strcpy(symmetric + 40, "...");
+	strcpy(symmetric + 43, sha1_to_hex(theirs->object.sha1));
+
+	init_revisions(&revs, NULL);
+	setup_revisions(rev_argc, rev_argv, &revs, NULL);
+	prepare_revision_walk(&revs);
+
+	/* ... and count the commits on each side. */
+	num_ours = 0;
+	num_theirs = 0;
+	while (1) {
+		struct commit *c = get_revision(&revs);
+		if (!c)
+			break;
+		if (c->object.flags & SYMMETRIC_LEFT)
+			num_ours++;
+		else
+			num_theirs++;
+	}
+
+	if (!prefixcmp(base, "refs/remotes/")) {
+		remote_msg = " remote";
+		base += strlen("refs/remotes/");
+	} else {
+		remote_msg = "";
+	}
+
+	if (!num_theirs)
+		printf("Your branch is ahead of the tracked%s branch '%s' "
+		       "by %d commit%s.\n",
+		       remote_msg, base,
+		       num_ours, (num_ours == 1) ? "" : "s");
+	else if (!num_ours)
+		printf("Your branch is behind the tracked%s branch '%s' "
+		       "by %d commit%s,\n"
+		       "and can be fast-forwarded.\n",
+		       remote_msg, base,
+		       num_theirs, (num_theirs == 1) ? "" : "s");
+	else
+		printf("Your branch and the tracked%s branch '%s' "
+		       "have diverged,\nand respectively "
+		       "have %d and %d different commit(s) each.\n",
+		       remote_msg, base,
+		       num_ours, num_theirs);
+}
+
+static void update_refs_for_switch(struct checkout_opts *opts,
+				   struct branch_info *old,
+				   struct branch_info *new)
+{
+	struct strbuf msg;
+	const char *old_desc;
+	if (opts->new_branch) {
+		create_branch(old->name, opts->new_branch, new->name, 0,
+			      opts->new_branch_log, opts->track);
+		new->name = opts->new_branch;
+		setup_branch_path(new);
+	}
+
+	strbuf_init(&msg, 0);
+	old_desc = old->name;
+	if (!old_desc)
+		old_desc = sha1_to_hex(old->commit->object.sha1);
+	strbuf_addf(&msg, "checkout: moving from %s to %s",
+		    old_desc, new->name);
+
+	if (new->path) {
+		create_symref("HEAD", new->path, msg.buf);
+		if (!opts->quiet) {
+			if (old->path && !strcmp(new->path, old->path))
+				fprintf(stderr, "Already on \"%s\"\n",
+					new->name);
+			else
+				fprintf(stderr, "Switched to%s branch \"%s\"\n",
+					opts->new_branch ? " a new" : "",
+					new->name);
+		}
+	} else if (strcmp(new->name, "HEAD")) {
+		update_ref(msg.buf, "HEAD", new->commit->object.sha1, NULL,
+			   REF_NODEREF, DIE_ON_ERR);
+		if (!opts->quiet) {
+			if (old->path)
+				fprintf(stderr, "Note: moving to \"%s\" which isn't a local branch\nIf you want to create a new branch from this checkout, you may do so\n(now or later) by using -b with the checkout command again. Example:\n  git checkout -b <new_branch_name>\n", new->name);
+			describe_detached_head("HEAD is now at", new->commit);
+		}
+	}
+	remove_branch_state();
+	strbuf_release(&msg);
+	if (!opts->quiet && (new->path || !strcmp(new->name, "HEAD")))
+		report_tracking(new, opts);
+}
+
+static int switch_branches(struct checkout_opts *opts, struct branch_info *new)
+{
+	int ret = 0;
+	struct branch_info old;
+	unsigned char rev[20];
+	int flag;
+	memset(&old, 0, sizeof(old));
+	old.path = resolve_ref("HEAD", rev, 0, &flag);
+	old.commit = lookup_commit_reference_gently(rev, 1);
+	if (!(flag & REF_ISSYMREF))
+		old.path = NULL;
+
+	if (old.path && !prefixcmp(old.path, "refs/heads/"))
+		old.name = old.path + strlen("refs/heads/");
+
+	if (!new->name) {
+		new->name = "HEAD";
+		new->commit = old.commit;
+		if (!new->commit)
+			die("You are on a branch yet to be born");
+		parse_commit(new->commit);
+	}
+
+	/*
+	 * If the new thing isn't a branch and isn't HEAD and we're
+	 * not starting a new branch, and we want messages, and we
+	 * weren't on a branch, and we're moving to a new commit,
+	 * describe the old commit.
+	 */
+	if (!new->path && strcmp(new->name, "HEAD") && !opts->new_branch &&
+	    !opts->quiet && !old.path && new->commit != old.commit)
+		describe_detached_head("Previous HEAD position was", old.commit);
+
+	if (!old.commit) {
+		if (!opts->quiet) {
+			fprintf(stderr, "warning: You appear to be on a branch yet to be born.\n");
+			fprintf(stderr, "warning: Forcing checkout of %s.\n", new->name);
+		}
+		opts->force = 1;
+	}
+
+	ret = merge_working_tree(opts, &old, new);
+	if (ret)
+		return ret;
+
+	update_refs_for_switch(opts, &old, new);
+
+	return post_checkout_hook(old.commit, new->commit, 1);
+}
+
+static int branch_track = 0;
+
+static int git_checkout_config(const char *var, const char *value)
+{
+	if (!strcmp(var, "branch.autosetupmerge"))
+		branch_track = git_config_bool(var, value);
+
+	return git_default_config(var, value);
+}
+
+int cmd_checkout(int argc, const char **argv, const char *prefix)
+{
+	struct checkout_opts opts;
+	unsigned char rev[20];
+	const char *arg;
+	struct branch_info new;
+	struct tree *source_tree = NULL;
+	struct option options[] = {
+		OPT__QUIET(&opts.quiet),
+		OPT_STRING('b', NULL, &opts.new_branch, "new branch", "branch"),
+		OPT_BOOLEAN('l', NULL, &opts.new_branch_log, "log for new branch"),
+		OPT_BOOLEAN( 0 , "track", &opts.track, "track"),
+		OPT_BOOLEAN('f', NULL, &opts.force, "force"),
+		OPT_BOOLEAN('m', NULL, &opts.merge, "merge"),
+		OPT_END(),
+	};
+
+	memset(&opts, 0, sizeof(opts));
+	memset(&new, 0, sizeof(new));
+
+	git_config(git_checkout_config);
+
+	opts.track = branch_track;
+
+	argc = parse_options(argc, argv, options, checkout_usage, 0);
+	if (argc) {
+		arg = argv[0];
+		if (get_sha1(arg, rev))
+			;
+		else if ((new.commit = lookup_commit_reference_gently(rev, 1))) {
+			new.name = arg;
+			setup_branch_path(&new);
+			if (resolve_ref(new.path, rev, 1, NULL))
+				new.commit = lookup_commit_reference(rev);
+			else
+				new.path = NULL;
+			parse_commit(new.commit);
+			source_tree = new.commit->tree;
+			argv++;
+			argc--;
+		} else if ((source_tree = parse_tree_indirect(rev))) {
+			argv++;
+			argc--;
+		}
+	}
+
+	if (argc && !strcmp(argv[0], "--")) {
+		argv++;
+		argc--;
+	}
+
+	if (!opts.new_branch && (opts.track != branch_track))
+		die("git checkout: --track and --no-track require -b");
+
+	if (opts.force && opts.merge)
+		die("git checkout: -f and -m are incompatible");
+
+	if (argc) {
+		const char **pathspec = get_pathspec(prefix, argv);
+		/* Checkout paths */
+		if (opts.new_branch || opts.force || opts.merge) {
+			if (argc == 1) {
+				die("git checkout: updating paths is incompatible with switching branches/forcing\nDid you intend to checkout '%s' which can not be resolved as commit?", argv[0]);
+			} else {
+				die("git checkout: updating paths is incompatible with switching branches/forcing");
+			}
+		}
+
+		if (source_tree)
+			read_tree_some(source_tree, pathspec);
+		else
+			read_cache();
+		return checkout_paths(pathspec);
+	}
+
+	if (new.name && !new.commit) {
+		die("Cannot switch branch to a non-commit.");
+	}
+
+	return switch_branches(&opts, &new);
+}
diff --git a/builtin-commit.c b/builtin-commit.c
index 065e1f7b7f..f49c22e642 100644
--- a/builtin-commit.c
+++ b/builtin-commit.c
@@ -205,7 +205,8 @@ static void create_base_index(void)
 		die("failed to unpack HEAD tree object");
 	parse_tree(tree);
 	init_tree_desc(&t, tree->buffer, tree->size);
-	unpack_trees(1, &t, &opts);
+	if (unpack_trees(1, &t, &opts))
+		exit(128); /* We've already reported the error, finish dying */
 }
 
 static char *prepare_index(int argc, const char **argv, const char *prefix)
diff --git a/merge-recursive.c b/builtin-merge-recursive.c
index 55ef76f5a5..6fe4102c0c 100644
--- a/merge-recursive.c
+++ b/builtin-merge-recursive.c
@@ -7,6 +7,7 @@
 #include "cache-tree.h"
 #include "commit.h"
 #include "blob.h"
+#include "builtin.h"
 #include "tree-walk.h"
 #include "diff.h"
 #include "diffcore.h"
@@ -17,6 +18,7 @@
 #include "xdiff-interface.h"
 #include "interpolate.h"
 #include "attr.h"
+#include "merge-recursive.h"
 
 static int subtree_merge;
 
@@ -221,22 +223,11 @@ static int git_merge_trees(int index_only,
 	return rc;