1 files changed, 230 insertions, 150 deletions

diff --git a/builtin/merge.c b/builtin/merge.c
index 3b0f8f96d4..41467e4277 100644
--- a/builtin/merge.c
+++ b/builtin/merge.c
@@ -64,6 +64,7 @@ static int option_renormalize;
 static int verbosity;
 static int allow_rerere_auto;
 static int abort_current_merge;
+static int allow_unrelated_histories;
 static int show_progress = -1;
 static int default_to_upstream = 1;
 static const char *sign_commit;
@@ -221,6 +222,8 @@ static struct option builtin_merge_options[] = {
 	OPT__VERBOSITY(&verbosity),
 	OPT_BOOL(0, "abort", &abort_current_merge,
 		N_("abort the current in-progress merge")),
+	OPT_BOOL(0, "allow-unrelated-histories", &allow_unrelated_histories,
+		 N_("allow merging unrelated histories")),
 	OPT_SET_INT(0, "progress", &show_progress, N_("force progress reporting"), 1),
 	{ OPTION_STRING, 'S', "gpg-sign", &sign_commit, N_("key-id"),
 	  N_("GPG sign commit"), PARSE_OPT_OPTARG, NULL, (intptr_t) "" },
@@ -231,9 +234,9 @@ static struct option builtin_merge_options[] = {
 /* Cleans up metadata that is uninteresting after a succeeded merge. */
 static void drop_save(void)
 {
-	unlink(git_path("MERGE_HEAD"));
-	unlink(git_path("MERGE_MSG"));
-	unlink(git_path("MERGE_MODE"));
+	unlink(git_path_merge_head());
+	unlink(git_path_merge_msg());
+	unlink(git_path_merge_mode());
 }
 
 static int save_state(unsigned char *stash)
@@ -338,7 +341,7 @@ static void squash_message(struct commit *commit, struct commit_list *remotehead
 	struct pretty_print_context ctx = {0};
 
 	printf(_("Squash commit -- not updating HEAD\n"));
-	filename = git_path("SQUASH_MSG");
+	filename = git_path_squash_msg();
 	fd = open(filename, O_WRONLY | O_CREAT, 0666);
 	if (fd < 0)
 		die_errno(_("Could not write to '%s'"), filename);
@@ -365,7 +368,7 @@ static void squash_message(struct commit *commit, struct commit_list *remotehead
 	while ((commit = get_revision(&rev)) != NULL) {
 		strbuf_addch(&out, '\n');
 		strbuf_addf(&out, "commit %s\n",
-			sha1_to_hex(commit->object.sha1));
+			oid_to_hex(&commit->object.oid));
 		pretty_print_commit(&ctx, commit, &out);
 	}
 	if (write_in_full(fd, out.buf, out.len) != out.len)
@@ -380,7 +383,7 @@ static void finish(struct commit *head_commit,
 		   const unsigned char *new_head, const char *msg)
 {
 	struct strbuf reflog_message = STRBUF_INIT;
-	const unsigned char *head = head_commit->object.sha1;
+	const unsigned char *head = head_commit->object.oid.hash;
 
 	if (!msg)
 		strbuf_addstr(&reflog_message, getenv("GIT_REFLOG_ACTION"));
@@ -404,6 +407,7 @@ static void finish(struct commit *head_commit,
 			 * We ignore errors in 'gc --auto', since the
 			 * user should see them.
 			 */
+			close_all_packs();
 			run_command_v_opt(argv_gc_auto, RUN_GIT_CMD);
 		}
 	}
@@ -492,40 +496,18 @@ static void merge_name(const char *remote, struct strbuf *msg)
 	}
 	if (len) {
 		struct strbuf truname = STRBUF_INIT;
-		strbuf_addstr(&truname, "refs/heads/");
-		strbuf_addstr(&truname, remote);
+		strbuf_addf(&truname, "refs/heads/%s", remote);
 		strbuf_setlen(&truname, truname.len - len);
 		if (ref_exists(truname.buf)) {
 			strbuf_addf(msg,
 				    "%s\t\tbranch '%s'%s of .\n",
-				    sha1_to_hex(remote_head->object.sha1),
+				    sha1_to_hex(remote_head->object.oid.hash),
 				    truname.buf + 11,
 				    (early ? " (early part)" : ""));
 			strbuf_release(&truname);
 			goto cleanup;
 		}
-	}
-
-	if (!strcmp(remote, "FETCH_HEAD") &&
-			!access(git_path("FETCH_HEAD"), R_OK)) {
-		const char *filename;
-		FILE *fp;
-		struct strbuf line = STRBUF_INIT;
-		char *ptr;
-
-		filename = git_path("FETCH_HEAD");
-		fp = fopen(filename, "r");
-		if (!fp)
-			die_errno(_("could not open '%s' for reading"),
-				  filename);
-		strbuf_getline(&line, fp, '\n');
-		fclose(fp);
-		ptr = strstr(line.buf, "\tnot-for-merge\t");
-		if (ptr)
-			strbuf_remove(&line, ptr-line.buf+1, 13);
-		strbuf_addbuf(msg, &line);
-		strbuf_release(&line);
-		goto cleanup;
+		strbuf_release(&truname);
 	}
 
 	if (remote_head->util) {
@@ -533,7 +515,7 @@ static void merge_name(const char *remote, struct strbuf *msg)
 		desc = merge_remote_util(remote_head);
 		if (desc && desc->obj && desc->obj->type == OBJ_TAG) {
 			strbuf_addf(msg, "%s\t\t%s '%s'\n",
-				    sha1_to_hex(desc->obj->sha1),
+				    sha1_to_hex(desc->obj->oid.hash),
 				    typename(desc->obj->type),
 				    remote);
 			goto cleanup;
@@ -541,7 +523,7 @@ static void merge_name(const char *remote, struct strbuf *msg)
 	}
 
 	strbuf_addf(msg, "%s\t\tcommit '%s'\n",
-		sha1_to_hex(remote_head->object.sha1), remote);
+		sha1_to_hex(remote_head->object.oid.hash), remote);
 cleanup:
 	strbuf_release(&buf);
 	strbuf_release(&bname);
@@ -776,7 +758,7 @@ static void add_strategies(const char *string, unsigned attr)
 
 static void write_merge_msg(struct strbuf *msg)
 {
-	const char *filename = git_path("MERGE_MSG");
+	const char *filename = git_path_merge_msg();
 	int fd = open(filename, O_WRONLY | O_CREAT, 0666);
 	if (fd < 0)
 		die_errno(_("Could not open '%s' for writing"),
@@ -788,7 +770,7 @@ static void write_merge_msg(struct strbuf *msg)
 
 static void read_merge_msg(struct strbuf *msg)
 {
-	const char *filename = git_path("MERGE_MSG");
+	const char *filename = git_path_merge_msg();
 	strbuf_reset(msg);
 	if (strbuf_read_file(msg, filename, 0) < 0)
 		die_errno(_("Could not read from '%s'"), filename);
@@ -821,14 +803,14 @@ static void prepare_to_commit(struct commit_list *remoteheads)
 		strbuf_commented_addf(&msg, _(merge_editor_comment), comment_line_char);
 	write_merge_msg(&msg);
 	if (run_commit_hook(0 < option_edit, get_index_file(), "prepare-commit-msg",
-			    git_path("MERGE_MSG"), "merge", NULL))
+			    git_path_merge_msg(), "merge", NULL))
 		abort_commit(remoteheads, NULL);
 	if (0 < option_edit) {
-		if (launch_editor(git_path("MERGE_MSG"), NULL, NULL))
+		if (launch_editor(git_path_merge_msg(), NULL, NULL))
 			abort_commit(remoteheads, NULL);
 	}
 	read_merge_msg(&msg);
-	stripspace(&msg, 0 < option_edit);
+	strbuf_stripspace(&msg, 0 < option_edit);
 	if (!msg.len)
 		abort_commit(remoteheads, _("Empty commit message."));
 	strbuf_release(&merge_msg);
@@ -887,7 +869,7 @@ static int suggest_conflicts(void)
 	FILE *fp;
 	struct strbuf msgbuf = STRBUF_INIT;
 
-	filename = git_path("MERGE_MSG");
+	filename = git_path_merge_msg();
 	fp = fopen(filename, "a");
 	if (!fp)
 		die_errno(_("Could not open '%s' for writing"), filename);
@@ -914,7 +896,7 @@ static struct commit *is_old_style_invocation(int argc, const char **argv,
 		second_token = lookup_commit_reference_gently(second_sha1, 0);
 		if (!second_token)
 			die(_("'%s' is not a commit"), argv[1]);
-		if (hashcmp(second_token->object.sha1, head))
+		if (hashcmp(second_token->object.oid.hash, head))
 			return NULL;
 	}
 	return second_token;
@@ -955,12 +937,12 @@ static int setup_with_upstream(const char ***argv)
 
 	if (!branch)
 		die(_("No current branch."));
-	if (!branch->remote)
+	if (!branch->remote_name)
 		die(_("No remote for the current branch."));
 	if (!branch->merge_nr)
 		die(_("No default upstream defined for the current branch."));
 
-	args = xcalloc(branch->merge_nr + 1, sizeof(char *));
+	args = xcalloc(st_add(branch->merge_nr, 1), sizeof(char *));
 	for (i = 0; i < branch->merge_nr; i++) {
 		if (!branch->merge[i]->dst)
 			die(_("No remote-tracking branch for %s from %s"),
@@ -980,16 +962,16 @@ static void write_merge_state(struct commit_list *remoteheads)
 	struct strbuf buf = STRBUF_INIT;
 
 	for (j = remoteheads; j; j = j->next) {
-		unsigned const char *sha1;
+		struct object_id *oid;
 		struct commit *c = j->item;
 		if (c->util && merge_remote_util(c)->obj) {
-			sha1 = merge_remote_util(c)->obj->sha1;
+			oid = &merge_remote_util(c)->obj->oid;
 		} else {
-			sha1 = c->object.sha1;
+			oid = &c->object.oid;
 		}
-		strbuf_addf(&buf, "%s\n", sha1_to_hex(sha1));
+		strbuf_addf(&buf, "%s\n", oid_to_hex(oid));
 	}
-	filename = git_path("MERGE_HEAD");
+	filename = git_path_merge_head();
 	fd = open(filename, O_WRONLY | O_CREAT, 0666);
 	if (fd < 0)
 		die_errno(_("Could not open '%s' for writing"), filename);
@@ -999,7 +981,7 @@ static void write_merge_state(struct commit_list *remoteheads)
 	strbuf_addch(&merge_msg, '\n');
 	write_merge_msg(&merge_msg);
 
-	filename = git_path("MERGE_MODE");
+	filename = git_path_merge_mode();
 	fd = open(filename, O_WRONLY | O_CREAT | O_TRUNC, 0666);
 	if (fd < 0)
 		die_errno(_("Could not open '%s' for writing"), filename);
@@ -1037,38 +1019,144 @@ static int default_edit_option(void)
 		st_stdin.st_mode == st_stdout.st_mode);
 }
 
+static struct commit_list *reduce_parents(struct commit *head_commit,
+					  int *head_subsumed,
+					  struct commit_list *remoteheads)
+{
+	struct commit_list *parents, **remotes;
+
+	/*
+	 * Is the current HEAD reachable from another commit being
+	 * merged?  If so we do not want to record it as a parent of
+	 * the resulting merge, unless --no-ff is given.  We will flip
+	 * this variable to 0 when we find HEAD among the independent
+	 * tips being merged.
+	 */
+	*head_subsumed = 1;
+
+	/* Find what parents to record by checking independent ones. */
+	parents = reduce_heads(remoteheads);
+
+	remoteheads = NULL;
+	remotes = &remoteheads;
+	while (parents) {
+		struct commit *commit = pop_commit(&parents);
+		if (commit == head_commit)
+			*head_subsumed = 0;
+		else
+			remotes = &commit_list_insert(commit, remotes)->next;
+	}
+	return remoteheads;
+}
+
+static void prepare_merge_message(struct strbuf *merge_names, struct strbuf *merge_msg)
+{
+	struct fmt_merge_msg_opts opts;
+
+	memset(&opts, 0, sizeof(opts));
+	opts.add_title = !have_message;
+	opts.shortlog_len = shortlog_len;
+	opts.credit_people = (0 < option_edit);
+
+	fmt_merge_msg(merge_names, merge_msg, &opts);
+	if (merge_msg->len)
+		strbuf_setlen(merge_msg, merge_msg->len - 1);
+}
+
+static void handle_fetch_head(struct commit_list **remotes, struct strbuf *merge_names)
+{
+	const char *filename;
+	int fd, pos, npos;
+	struct strbuf fetch_head_file = STRBUF_INIT;
+
+	if (!merge_names)
+		merge_names = &fetch_head_file;
+
+	filename = git_path_fetch_head();
+	fd = open(filename, O_RDONLY);
+	if (fd < 0)
+		die_errno(_("could not open '%s' for reading"), filename);
+
+	if (strbuf_read(merge_names, fd, 0) < 0)
+		die_errno(_("could not read '%s'"), filename);
+	if (close(fd) < 0)
+		die_errno(_("could not close '%s'"), filename);
+
+	for (pos = 0; pos < merge_names->len; pos = npos) {
+		unsigned char sha1[20];
+		char *ptr;
+		struct commit *commit;
+
+		ptr = strchr(merge_names->buf + pos, '\n');
+		if (ptr)
+			npos = ptr - merge_names->buf + 1;
+		else
+			npos = merge_names->len;
+
+		if (npos - pos < 40 + 2 ||
+		    get_sha1_hex(merge_names->buf + pos, sha1))
+			commit = NULL; /* bad */
+		else if (memcmp(merge_names->buf + pos + 40, "\t\t", 2))
+			continue; /* not-for-merge */
+		else {
+			char saved = merge_names->buf[pos + 40];
+			merge_names->buf[pos + 40] = '\0';
+			commit = get_merge_parent(merge_names->buf + pos);
+			merge_names->buf[pos + 40] = saved;
+		}
+		if (!commit) {
+			if (ptr)
+				*ptr = '\0';
+			die("not something we can merge in %s: %s",
+			    filename, merge_names->buf + pos);
+		}
+		remotes = &commit_list_insert(commit, remotes)->next;
+	}
+
+	if (merge_names == &fetch_head_file)
+		strbuf_release(&fetch_head_file);
+}
+
 static struct commit_list *collect_parents(struct commit *head_commit,
 					   int *head_subsumed,
-					   int argc, const char **argv)
+					   int argc, const char **argv,
+					   struct strbuf *merge_msg)
 {
 	int i;
-	struct commit_list *remoteheads = NULL, *parents, *next;
+	struct commit_list *remoteheads = NULL;
 	struct commit_list **remotes = &remoteheads;
+	struct strbuf merge_names = STRBUF_INIT, *autogen = NULL;
+
+	if (merge_msg && (!have_message || shortlog_len))
+		autogen = &merge_names;
 
 	if (head_commit)
 		remotes = &commit_list_insert(head_commit, remotes)->next;
-	for (i = 0; i < argc; i++) {
-		struct commit *commit = get_merge_parent(argv[i]);
-		if (!commit)
-			help_unknown_ref(argv[i], "merge",
-					 "not something we can merge");
-		remotes = &commit_list_insert(commit, remotes)->next;
-	}
-	*remotes = NULL;
-
-	parents = reduce_heads(remoteheads);
 
-	*head_subsumed = 1; /* we will flip this to 0 when we find it */
-	for (remoteheads = NULL, remotes = &remoteheads;
-	     parents;
-	     parents = next) {
-		struct commit *commit = parents->item;
-		next = parents->next;
-		if (commit == head_commit)
-			*head_subsumed = 0;
-		else
+	if (argc == 1 && !strcmp(argv[0], "FETCH_HEAD")) {
+		handle_fetch_head(remotes, autogen);
+		remoteheads = reduce_parents(head_commit, head_subsumed, remoteheads);
+	} else {
+		for (i = 0; i < argc; i++) {
+			struct commit *commit = get_merge_parent(argv[i]);
+			if (!commit)
+				help_unknown_ref(argv[i], "merge",
+						 "not something we can merge");
 			remotes = &commit_list_insert(commit, remotes)->next;
+		}
+		remoteheads = reduce_parents(head_commit, head_subsumed, remoteheads);
+		if (autogen) {
+			struct commit_list *p;
+			for (p = remoteheads; p; p = p->next)
+				merge_name(merge_remote_util(p->item)->name, autogen);
+		}
 	}
+
+	if (autogen) {
+		prepare_merge_message(autogen, merge_msg);
+		strbuf_release(autogen);
+	}
+
 	return remoteheads;
 }
 
@@ -1102,6 +1190,7 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 	else
 		head_commit = lookup_commit_or_die(head_sha1, "HEAD");
 
+	init_diff_ui_defaults();
 	git_config(git_merge_config, NULL);
 
 	if (branch_mergeoptions)
@@ -1118,7 +1207,7 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 		int nargc = 2;
 		const char *nargv[] = {"reset", "--merge", NULL};
 
-		if (!file_exists(git_path("MERGE_HEAD")))
+		if (!file_exists(git_path_merge_head()))
 			die(_("There is no merge to abort (MERGE_HEAD missing)."));
 
 		/* Invoke 'git reset --merge' */
@@ -1129,7 +1218,7 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 	if (read_cache_unmerged())
 		die_resolve_conflict("merge");
 
-	if (file_exists(git_path("MERGE_HEAD"))) {
+	if (file_exists(git_path_merge_head())) {
 		/*
 		 * There is no unmerged entry, don't advise 'git
 		 * add/rm <file>', just 'git commit'.
@@ -1140,7 +1229,7 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 		else
 			die(_("You have not concluded your merge (MERGE_HEAD exists)."));
 	}
-	if (file_exists(git_path("CHERRY_PICK_HEAD"))) {
+	if (file_exists(git_path_cherry_pick_head())) {
 		if (advice_resolve_conflict)
 			die(_("You have not concluded your cherry-pick (CHERRY_PICK_HEAD exists).\n"
 			    "Please, commit your changes before you merge."));
@@ -1158,61 +1247,62 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 		option_commit = 0;
 	}
 
-	if (!abort_current_merge) {
-		if (!argc) {
-			if (default_to_upstream)
-				argc = setup_with_upstream(&argv);
-			else
-				die(_("No commit specified and merge.defaultToUpstream not set."));
-		} else if (argc == 1 && !strcmp(argv[0], "-"))
-			argv[0] = "@{-1}";
+	if (!argc) {
+		if (default_to_upstream)
+			argc = setup_with_upstream(&argv);
+		else
+			die(_("No commit specified and merge.defaultToUpstream not set."));
+	} else if (argc == 1 && !strcmp(argv[0], "-")) {
+		argv[0] = "@{-1}";
 	}
+
 	if (!argc)
 		usage_with_options(builtin_merge_usage,
 			builtin_merge_options);
 
-	/*
-	 * This could be traditional "merge <msg> HEAD <commit>..."  and
-	 * the way we can tell it is to see if the second token is HEAD,
-	 * but some people might have misused the interface and used a
-	 * commit-ish that is the same as HEAD there instead.
-	 * Traditional format never would have "-m" so it is an
-	 * additional safety measure to check for it.
-	 */
-
-	if (!have_message && head_commit &&
-	    is_old_style_invocation(argc, argv, head_commit->object.sha1)) {
-		strbuf_addstr(&merge_msg, argv[0]);
-		head_arg = argv[1];
-		argv += 2;
-		argc -= 2;
-		remoteheads = collect_parents(head_commit, &head_subsumed, argc, argv);
-	} else if (!head_commit) {
-		struct commit *remote_head;
+	if (!head_commit) {
 		/*
 		 * If the merged head is a valid one there is no reason
 		 * to forbid "git merge" into a branch yet to be born.
 		 * We do the same for "git pull".
 		 */
-		if (argc != 1)
-			die(_("Can merge only exactly one commit into "
-				"empty head"));
+		unsigned char *remote_head_sha1;
 		if (squash)
 			die(_("Squash commit into empty head not supported yet"));
 		if (fast_forward == FF_NO)
 			die(_("Non-fast-forward commit does not make sense into "
 			    "an empty head"));
-		remoteheads = collect_parents(head_commit, &head_subsumed, argc, argv);
-		remote_head = remoteheads->item;
-		if (!remote_head)
+		remoteheads = collect_parents(head_commit, &head_subsumed,
+					      argc, argv, NULL);
+		if (!remoteheads)
 			die(_("%s - not something we can merge"), argv[0]);
-		read_empty(remote_head->object.sha1, 0);
-		update_ref("initial pull", "HEAD", remote_head->object.sha1,
+		if (remoteheads->next)
+			die(_("Can merge only exactly one commit into empty head"));
+		remote_head_sha1 = remoteheads->item->object.oid.hash;
+		read_empty(remote_head_sha1, 0);
+		update_ref("initial pull", "HEAD", remote_head_sha1,
 			   NULL, 0, UPDATE_REFS_DIE_ON_ERR);
 		goto done;
-	} else {
-		struct strbuf merge_names = STRBUF_INIT;
+	}
 
+	/*
+	 * This could be traditional "merge <msg> HEAD <commit>..."  and
+	 * the way we can tell it is to see if the second token is HEAD,
+	 * but some people might have misused the interface and used a
+	 * commit-ish that is the same as HEAD there instead.
+	 * Traditional format never would have "-m" so it is an
+	 * additional safety measure to check for it.
+	 */
+	if (!have_message &&
+	    is_old_style_invocation(argc, argv, head_commit->object.oid.hash)) {
+		warning("old-style 'git merge <msg> HEAD <commit>' is deprecated.");
+		strbuf_addstr(&merge_msg, argv[0]);
+		head_arg = argv[1];
+		argv += 2;
+		argc -= 2;
+		remoteheads = collect_parents(head_commit, &head_subsumed,
+					      argc, argv, NULL);
+	} else {
 		/* We are invoked directly as the first-class UI. */
 		head_arg = "HEAD";
 
@@ -1221,21 +1311,8 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 		 * the standard merge summary message to be appended
 		 * to the given message.
 		 */
-		remoteheads = collect_parents(head_commit, &head_subsumed, argc, argv);
-		for (p = remoteheads; p; p = p->next)
-			merge_name(merge_remote_util(p->item)->name, &merge_names);
-
-		if (!have_message || shortlog_len) {
-			struct fmt_merge_msg_opts opts;
-			memset(&opts, 0, sizeof(opts));
-			opts.add_title = !have_message;
-			opts.shortlog_len = shortlog_len;
-			opts.credit_people = (0 < option_edit);
-
-			fmt_merge_msg(&merge_names, &merge_msg, &opts);
-			if (merge_msg.len)
-				strbuf_setlen(&merge_msg, merge_msg.len - 1);
-		}
+		remoteheads = collect_parents(head_commit, &head_subsumed,
+					      argc, argv, &merge_msg);
 	}
 
 	if (!head_commit || !argc)
@@ -1245,13 +1322,13 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 	if (verify_signatures) {
 		for (p = remoteheads; p; p = p->next) {
 			struct commit *commit = p->item;
-			char hex[41];
+			char hex[GIT_SHA1_HEXSZ + 1];
 			struct signature_check signature_check;
 			memset(&signature_check, 0, sizeof(signature_check));
 
 			check_commit_signature(commit, &signature_check);
 
-			strcpy(hex, find_unique_abbrev(commit->object.sha1, DEFAULT_ABBREV));
+			find_unique_abbrev_r(hex, commit->object.oid.hash, DEFAULT_ABBREV);
 			switch (signature_check.result) {
 			case 'G':
 				break;
@@ -1281,7 +1358,7 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 	for (p = remoteheads; p; p = p->next) {
 		struct commit *commit = p->item;
 		strbuf_addf(&buf, "GITHEAD_%s",
-			    sha1_to_hex(commit->object.sha1));
+			    sha1_to_hex(commit->object.oid.hash));
 		setenv(buf.buf, merge_remote_util(commit)->name, 1);
 		strbuf_reset(&buf);
 		if (fast_forward != FF_ONLY &&
@@ -1321,12 +1398,15 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 		free(list);
 	}
 
-	update_ref("updating ORIG_HEAD", "ORIG_HEAD", head_commit->object.sha1,
+	update_ref("updating ORIG_HEAD", "ORIG_HEAD", head_commit->object.oid.hash,
 		   NULL, 0, UPDATE_REFS_DIE_ON_ERR);
 
-	if (remoteheads && !common)
-		; /* No common ancestors found. We need a real merge. */
-	else if (!remoteheads ||
+	if (remoteheads && !common) {
+		/* No common ancestors found. */
+		if (!allow_unrelated_histories)
+			die(_("refusing to merge unrelated histories"));
+		/* otherwise, we need a real merge. */
+	} else if (!remoteheads ||
 		 (!remoteheads->next && !common->next &&
 		  common->item == remoteheads->item)) {
 		/*
@@ -1337,19 +1417,19 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 		goto done;
 	} else if (fast_forward != FF_NO && !remoteheads->next &&
 			!common->next &&
-			!hashcmp(common->item->object.sha1, head_commit->object.sha1)) {
+			!hashcmp(common->item->object.oid.hash, head_commit->object.oid.hash)) {
 		/* Again the most common case of merging one remote. */
 		struct strbuf msg = STRBUF_INIT;
 		struct commit *commit;
-		char hex[41];
 
-		strcpy(hex, find_unique_abbrev(head_commit->object.sha1, DEFAULT_ABBREV));
-
-		if (verbosity >= 0)
-			printf(_("Updating %s..%s\n"),
-				hex,
-				find_unique_abbrev(remoteheads->item->object.sha1,
-				DEFAULT_ABBREV));
+		if (verbosity >= 0) {
+			char from[GIT_SHA1_HEXSZ + 1], to[GIT_SHA1_HEXSZ + 1];
+			find_unique_abbrev_r(from, head_commit->object.oid.hash,
+					      DEFAULT_ABBREV);
+			find_unique_abbrev_r(to, remoteheads->item->object.oid.hash,
+					      DEFAULT_ABBREV);
+			printf(_("Updating %s..%s\n"), from, to);
+		}
 		strbuf_addstr(&msg, "Fast-forward");
 		if (have_message)
 			strbuf_addstr(&msg,
@@ -1360,14 +1440,14 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 			goto done;
 		}
 
-		if (checkout_fast_forward(head_commit->object.sha1,
-					  commit->object.sha1,
+		if (checkout_fast_forward(head_commit->object.oid.hash,
+					  commit->object.oid.hash,
 					  overwrite_ignore)) {
 			ret = 1;
 			goto done;
 		}
 
-		finish(head_commit, remoteheads, commit->object.sha1, msg.buf);
+		finish(head_commit, remoteheads, commit->object.oid.hash, msg.buf);
 		drop_save();
 		goto done;
 	} else if (!remoteheads->next && common->next)
@@ -1386,9 +1466,9 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 			/* See if it is really trivial. */
 			git_committer_info(IDENT_STRICT);
 			printf(_("Trying really trivial in-index merge...\n"));
-			if (!read_tree_trivial(common->item->object.sha1,
-					       head_commit->object.sha1,
-					       remoteheads->item->object.sha1)) {
+			if (!read_tree_trivial(common->item->object.oid.hash,
+					       head_commit->object.oid.hash,
+					       remoteheads->item->object.oid.hash)) {
 				ret = merge_trivial(head_commit, remoteheads);
 				goto done;
 			}
@@ -1411,8 +1491,8 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 			 * HEAD^^" would be missed.
 			 */
 			common_one = get_merge_bases(head_commit, j->item);
-			if (hashcmp(common_one->item->object.sha1,
-				j->item->object.sha1)) {
+			if (hashcmp(common_one->item->object.oid.hash,
+				j->item->object.oid.hash)) {
 				up_to_date = 0;
 				break;
 			}
@@ -1448,7 +1528,7 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 		int ret;
 		if (i) {
 			printf(_("Rewinding the tree to pristine...\n"));
-			restore_state(head_commit->object.sha1, stash);
+			restore_state(head_commit->object.oid.hash, stash);
 		}
 		if (use_strategies_nr != 1)
 			printf(_("Trying merge strategy %s...\n"),
@@ -1514,7 +1594,7 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 	 * it up.
 	 */
 	if (!best_strategy) {
-		restore_state(head_commit->object.sha1, stash);
+		restore_state(head_commit->object.oid.hash, stash);
 		if (use_strategies_nr > 1)
 			fprintf(stderr,
 				_("No merge strategy handled the merge.\n"));
@@ -1527,7 +1607,7 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 		; /* We already have its result in the working tree. */
 	else {
 		printf(_("Rewinding the tree to pristine...\n"));
-		restore_state(head_commit->object.sha1, stash);
+		restore_state(head_commit->object.oid.hash, stash);
 		printf(_("Using the %s to prepare resolving by hand.\n"),
 			best_strategy);
 		try_merge_strategy(best_strategy, common, remoteheads,