Merge branch 'jn/merge-diff3-label'

* jn/merge-diff3-label:
  merge-recursive: add a label for ancestor
  cherry-pick, revert: add a label for ancestor
  revert: clarify label on conflict hunks
  compat: add mempcpy()
  checkout -m --conflict=diff3: add a label for ancestor
  merge_trees(): add ancestor label parameter for diff3-style output
  merge_file(): add comment explaining behavior wrt conflict style
  checkout --conflict=diff3: add a label for ancestor
  ll_merge(): add ancestor label parameter for diff3-style output
  merge-file --diff3: add a label for ancestor
  xdl_merge(): move file1 and file2 labels to xmparam structure
  xdl_merge(): add optional ancestor label to diff3-style output
  tests: document cherry-pick behavior in face of conflicts
  tests: document format of conflicts from checkout -m

Conflicts:
	builtin/revert.c

15 files changed, 405 insertions, 79 deletions

diff --git a/builtin/checkout.c b/builtin/checkout.c
index acefaaf41a..88b1f43e05 100644
--- a/builtin/checkout.c
+++ b/builtin/checkout.c
@@ -149,7 +149,7 @@ static int checkout_merged(int pos, struct checkout *state)
 	read_mmblob(&ours, active_cache[pos+1]->sha1);
 	read_mmblob(&theirs, active_cache[pos+2]->sha1);
 
-	status = ll_merge(&result_buf, path, &ancestor,
+	status = ll_merge(&result_buf, path, &ancestor, "base",
 			  &ours, "ours", &theirs, "theirs", 0);
 	free(ancestor.ptr);
 	free(ours.ptr);
@@ -439,6 +439,7 @@ static int merge_working_tree(struct checkout_opts *opts,
 			ret = reset_tree(new->commit->tree, opts, 1);
 			if (ret)
 				return ret;
+			o.ancestor = old->name;
 			o.branch1 = new->name;
 			o.branch2 = "local";
 			merge_trees(&o, new->commit->tree, work,
diff --git a/builtin/merge-file.c b/builtin/merge-file.c
index 69cc683332..610849a653 100644
--- a/builtin/merge-file.c
+++ b/builtin/merge-file.c
@@ -77,8 +77,10 @@ int cmd_merge_file(int argc, const char **argv, const char *prefix)
 					argv[i]);
 	}
 
-	ret = xdl_merge(mmfs + 1, mmfs + 0, names[0], mmfs + 2, names[2],
-			&xmp, &result);
+	xmp.ancestor = names[1];
+	xmp.file1 = names[0];
+	xmp.file2 = names[2];
+	ret = xdl_merge(mmfs + 1, mmfs + 0, mmfs + 2, &xmp, &result);
 
 	for (i = 0; i < 3; i++)
 		free(mmfs[i].ptr);
diff --git a/builtin/revert.c b/builtin/revert.c
index 9a3c14c329..778a56eb51 100644
--- a/builtin/revert.c
+++ b/builtin/revert.c
@@ -46,6 +46,8 @@ static const char *me;
 
 #define GIT_REFLOG_ACTION "GIT_REFLOG_ACTION"
 
+static char *get_encoding(const char *message);
+
 static void parse_args(int argc, const char **argv)
 {
 	const char * const * usage_str =
@@ -85,33 +87,64 @@ static void parse_args(int argc, const char **argv)
 		exit(1);
 }
 
-static char *get_oneline(const char *message)
+struct commit_message {
+	char *parent_label;
+	const char *label;
+	const char *subject;
+	char *reencoded_message;
+	const char *message;
+};
+
+static int get_message(const char *raw_message, struct commit_message *out)
 {
-	char *result;
-	const char *p = message, *abbrev, *eol;
+	const char *encoding;
+	const char *p, *abbrev, *eol;
+	char *q;
 	int abbrev_len, oneline_len;
 
-	if (!p)
-		die ("Could not read commit message of %s",
-				sha1_to_hex(commit->object.sha1));
+	if (!raw_message)
+		return -1;
+	encoding = get_encoding(raw_message);
+	if (!encoding)
+		encoding = "UTF-8";
+	if (!git_commit_encoding)
+		git_commit_encoding = "UTF-8";
+	if ((out->reencoded_message = reencode_string(raw_message,
+					git_commit_encoding, encoding)))
+		out->message = out->reencoded_message;
+
+	abbrev = find_unique_abbrev(commit->object.sha1, DEFAULT_ABBREV);
+	abbrev_len = strlen(abbrev);
+
+	/* Find beginning and end of commit subject. */
+	p = out->message;
 	while (*p && (*p != '\n' || p[1] != '\n'))
 		p++;
-
 	if (*p) {
 		p += 2;
 		for (eol = p + 1; *eol && *eol != '\n'; eol++)
 			; /* do nothing */
 	} else
 		eol = p;
-	abbrev = find_unique_abbrev(commit->object.sha1, DEFAULT_ABBREV);
-	abbrev_len = strlen(abbrev);
 	oneline_len = eol - p;
-	result = xmalloc(abbrev_len + 5 + oneline_len);
-	memcpy(result, abbrev, abbrev_len);
-	memcpy(result + abbrev_len, "... ", 4);
-	memcpy(result + abbrev_len + 4, p, oneline_len);
-	result[abbrev_len + 4 + oneline_len] = '\0';
-	return result;
+
+	out->parent_label = xmalloc(strlen("parent of ") + abbrev_len +
+			      strlen("... ") + oneline_len + 1);
+	q = out->parent_label;
+	q = mempcpy(q, "parent of ", strlen("parent of "));
+	out->label = q;
+	q = mempcpy(q, abbrev, abbrev_len);
+	q = mempcpy(q, "... ", strlen("... "));
+	out->subject = q;
+	q = mempcpy(q, p, oneline_len);
+	*q = '\0';
+	return 0;
+}
+
+static void free_message(struct commit_message *msg)
+{
+	free(msg->parent_label);
+	free(msg->reencoded_message);
 }
 
 static char *get_encoding(const char *message)
@@ -271,9 +304,9 @@ static int revert_or_cherry_pick(int argc, const char **argv)
 {
 	unsigned char head[20];
 	struct commit *base, *next, *parent;
+	const char *base_label, *next_label;
 	int i, index_fd, clean;
-	char *oneline, *reencoded_message = NULL;
-	const char *message, *encoding;
+	struct commit_message msg = { NULL, NULL, NULL, NULL, NULL };
 	char *defmsg = NULL;
 	struct merge_options o;
 	struct tree *result, *next_tree, *base_tree, *head_tree;
@@ -349,14 +382,14 @@ static int revert_or_cherry_pick(int argc, const char **argv)
 	if (allow_ff && !hashcmp(parent->object.sha1, head))
 		return fast_forward_to(commit->object.sha1, head);
 
-	if (!(message = commit->buffer))
-		die ("Cannot get commit message for %s",
-				sha1_to_hex(commit->object.sha1));
-
 	if (parent && parse_commit(parent) < 0)
 		die("%s: cannot parse parent commit %s",
 		    me, sha1_to_hex(parent->object.sha1));
 
+	if (get_message(commit->buffer, &msg) != 0)
+		die("Cannot get commit message for %s",
+				sha1_to_hex(commit->object.sha1));
+
 	/*
 	 * "commit" is an existing commit.  We would want to apply
 	 * the difference it introduces since its first parent "prev"
@@ -368,26 +401,15 @@ static int revert_or_cherry_pick(int argc, const char **argv)
 	msg_fd = hold_lock_file_for_update(&msg_file, defmsg,
 					   LOCK_DIE_ON_ERROR);
 
-	encoding = get_encoding(message);
-	if (!encoding)
-		encoding = "UTF-8";
-	if (!git_commit_encoding)
-		git_commit_encoding = "UTF-8";
-	if ((reencoded_message = reencode_string(message,
-					git_commit_encoding, encoding)))
-		message = reencoded_message;
-
-	oneline = get_oneline(message);
-
 	index_fd = hold_locked_index(&index_lock, 1);
 
 	if (action == REVERT) {
-		char *oneline_body = strchr(oneline, ' ');
-
 		base = commit;
+		base_label = msg.label;
 		next = parent;
+		next_label = msg.parent_label;
 		add_to_msg("Revert \"");
-		add_to_msg(oneline_body + 1);
+		add_to_msg(msg.subject);
 		add_to_msg("\"\n\nThis reverts commit ");
 		add_to_msg(sha1_to_hex(commit->object.sha1));
 
@@ -398,9 +420,11 @@ static int revert_or_cherry_pick(int argc, const char **argv)
 		add_to_msg(".\n");
 	} else {
 		base = parent;
+		base_label = msg.parent_label;
 		next = commit;
-		set_author_ident_env(message);
-		add_message_to_msg(message);
+		next_label = msg.label;
+		set_author_ident_env(msg.message);
+		add_message_to_msg(msg.message);
 		if (no_replay) {
 			add_to_msg("(cherry picked from commit ");
 			add_to_msg(sha1_to_hex(commit->object.sha1));
@@ -410,8 +434,9 @@ static int revert_or_cherry_pick(int argc, const char **argv)
 
 	read_cache();
 	init_merge_options(&o);
+	o.ancestor = base ? base_label : "(empty tree)";
 	o.branch1 = "HEAD";
-	o.branch2 = oneline;
+	o.branch2 = next ? next_label : "(empty tree)";
 
 	head_tree = parse_tree_indirect(head);
 	next_tree = next ? next->tree : empty_tree();
@@ -475,7 +500,7 @@ static int revert_or_cherry_pick(int argc, const char **argv)
 		args[i] = NULL;
 		return execv_git_cmd(args);
 	}
-	free(reencoded_message);
+	free_message(&msg);
 	free(defmsg);
 
 	return 0;
diff --git a/git-compat-util.h b/git-compat-util.h
index e292926fec..7e62b55270 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -332,6 +332,7 @@ extern int git_vsnprintf(char *str, size_t maxsize,
 #ifdef __GLIBC_PREREQ
 #if __GLIBC_PREREQ(2, 1)
 #define HAVE_STRCHRNUL
+#define HAVE_MEMPCPY
 #endif
 #endif
 
@@ -345,6 +346,14 @@ static inline char *gitstrchrnul(const char *s, int c)
 }
 #endif
 
+#ifndef HAVE_MEMPCPY
+#define mempcpy gitmempcpy
+static inline void *gitmempcpy(void *dest, const void *src, size_t n)
+{
+	return (char *)memcpy(dest, src, n) + n;
+}
+#endif
+
 extern void release_pack_memory(size_t, int);
 
 extern char *xstrdup(const char *str);
diff --git a/ll-merge.c b/ll-merge.c
index 82c7742e41..f9b3d854a9 100644
--- a/ll-merge.c
+++ b/ll-merge.c
@@ -15,7 +15,7 @@ struct ll_merge_driver;
 typedef int (*ll_merge_fn)(const struct ll_merge_driver *,
 			   mmbuffer_t *result,
 			   const char *path,
-			   mmfile_t *orig,
+			   mmfile_t *orig, const char *orig_name,
 			   mmfile_t *src1, const char *name1,
 			   mmfile_t *src2, const char *name2,
 			   int flag,
@@ -36,7 +36,7 @@ struct ll_merge_driver {
 static int ll_binary_merge(const struct ll_merge_driver *drv_unused,
 			   mmbuffer_t *result,
 			   const char *path_unused,
-			   mmfile_t *orig,
+			   mmfile_t *orig, const char *orig_name,
 			   mmfile_t *src1, const char *name1,
 			   mmfile_t *src2, const char *name2,
 			   int flag, int marker_size)
@@ -57,7 +57,7 @@ static int ll_binary_merge(const struct ll_merge_driver *drv_unused,
 static int ll_xdl_merge(const struct ll_merge_driver *drv_unused,
 			mmbuffer_t *result,
 			const char *path,
-			mmfile_t *orig,
+			mmfile_t *orig, const char *orig_name,
 			mmfile_t *src1, const char *name1,
 			mmfile_t *src2, const char *name2,
 			int flag, int marker_size)
@@ -71,7 +71,8 @@ static int ll_xdl_merge(const struct ll_merge_driver *drv_unused,
 			path, name1, name2);
 		return ll_binary_merge(drv_unused, result,
 				       path,
-				       orig, src1, name1,
+				       orig, orig_name,
+				       src1, name1,
 				       src2, name2,
 				       flag, marker_size);
 	}
@@ -83,13 +84,16 @@ static int ll_xdl_merge(const struct ll_merge_driver *drv_unused,
 		xmp.style = git_xmerge_style;
 	if (marker_size > 0)
 		xmp.marker_size = marker_size;
-	return xdl_merge(orig, src1, name1, src2, name2, &xmp, result);
+	xmp.ancestor = orig_name;
+	xmp.file1 = name1;
+	xmp.file2 = name2;
+	return xdl_merge(orig, src1, src2, &xmp, result);
 }
 
 static int ll_union_merge(const struct ll_merge_driver *drv_unused,
 			  mmbuffer_t *result,
 			  const char *path_unused,
-			  mmfile_t *orig,
+			  mmfile_t *orig, const char *orig_name,
 			  mmfile_t *src1, const char *name1,
 			  mmfile_t *src2, const char *name2,
 			  int flag, int marker_size)
@@ -97,7 +101,7 @@ static int ll_union_merge(const struct ll_merge_driver *drv_unused,
 	/* Use union favor */
 	flag = (flag & 1) | (XDL_MERGE_FAVOR_UNION << 1);
 	return ll_xdl_merge(drv_unused, result, path_unused,
-			    orig, src1, NULL, src2, NULL,
+			    orig, NULL, src1, NULL, src2, NULL,
 			    flag, marker_size);
 	return 0;
 }
@@ -128,7 +132,7 @@ static void create_temp(mmfile_t *src, char *path)
 static int ll_ext_merge(const struct ll_merge_driver *fn,
 			mmbuffer_t *result,
 			const char *path,
-			mmfile_t *orig,
+			mmfile_t *orig, const char *orig_name,
 			mmfile_t *src1, const char *name1,
 			mmfile_t *src2, const char *name2,
 			int flag, int marker_size)
@@ -319,7 +323,7 @@ static int git_path_check_merge(const char *path, struct git_attr_check check[2]
 
 int ll_merge(mmbuffer_t *result_buf,
 	     const char *path,
-	     mmfile_t *ancestor,
+	     mmfile_t *ancestor, const char *ancestor_label,
 	     mmfile_t *ours, const char *our_label,
 	     mmfile_t *theirs, const char *their_label,
 	     int flag)
@@ -341,7 +345,7 @@ int ll_merge(mmbuffer_t *result_buf,
 	driver = find_ll_merge_driver(ll_driver_name);
 	if (virtual_ancestor && driver->recursive)
 		driver = find_ll_merge_driver(driver->recursive);
-	return driver->fn(driver, result_buf, path, ancestor,
+	return driver->fn(driver, result_buf, path, ancestor, ancestor_label,
 			  ours, our_label, theirs, their_label,
 			  flag, marker_size);
 }
diff --git a/ll-merge.h b/ll-merge.h
index 57889227b1..57754cc8ca 100644
--- a/ll-merge.h
+++ b/ll-merge.h
@@ -7,7 +7,7 @@
 
 int ll_merge(mmbuffer_t *result_buf,
 	     const char *path,
-	     mmfile_t *ancestor,
+	     mmfile_t *ancestor, const char *ancestor_label,
 	     mmfile_t *ours, const char *our_label,
 	     mmfile_t *theirs, const char *their_label,
 	     int flag);
diff --git a/merge-file.c b/merge-file.c
index fd34d76e15..c336c93c01 100644
--- a/merge-file.c
+++ b/merge-file.c
@@ -30,7 +30,13 @@ static void *three_way_filemerge(const char *path, mmfile_t *base, mmfile_t *our
 	int merge_status;
 	mmbuffer_t res;
 
-	merge_status = ll_merge(&res, path, base,
+	/*
+	 * This function is only used by cmd_merge_tree, which
+	 * does not respect the merge.conflictstyle option.
+	 * There is no need to worry about a label for the
+	 * common ancestor.
+	 */
+	merge_status = ll_merge(&res, path, base, NULL,
 				our, ".our", their, ".their", 0);
 	if (merge_status < 0)
 		return NULL;
diff --git a/merge-recursive.c b/merge-recursive.c
index 195ebf9744..917397ca7a 100644
--- a/merge-recursive.c
+++ b/merge-recursive.c
@@ -608,7 +608,7 @@ static int merge_3way(struct merge_options *o,
 		      const char *branch2)
 {
 	mmfile_t orig, src1, src2;
-	char *name1, *name2;
+	char *base_name, *name1, *name2;
 	int merge_status;
 	int favor;
 
@@ -628,10 +628,15 @@ static int merge_3way(struct merge_options *o,
 		}
 	}
 
-	if (strcmp(a->path, b->path)) {
+	if (strcmp(a->path, b->path) ||
+	    (o->ancestor != NULL && strcmp(a->path, one->path) != 0)) {
+		base_name = o->ancestor == NULL ? NULL :
+			xstrdup(mkpath("%s:%s", o->ancestor, one->path));
 		name1 = xstrdup(mkpath("%s:%s", branch1, a->path));
 		name2 = xstrdup(mkpath("%s:%s", branch2, b->path));
 	} else {
+		base_name = o->ancestor == NULL ? NULL :
+			xstrdup(mkpath("%s", o->ancestor));
 		name1 = xstrdup(mkpath("%s", branch1));
 		name2 = xstrdup(mkpath("%s", branch2));
 	}
@@ -640,7 +645,7 @@ static int merge_3way(struct merge_options *o,
 	read_mmblob(&src1, a->sha1);
 	read_mmblob(&src2, b->sha1);
 
-	merge_status = ll_merge(result_buf, a->path, &orig,
+	merge_status = ll_merge(result_buf, a->path, &orig, base_name,
 				&src1, name1, &src2, name2,
 				(!!o->call_depth) | (favor << 1));
 
@@ -1342,6 +1347,7 @@ int merge_recursive(struct merge_options *o,
 	if (!o->call_depth)
 		read_cache();
 
+	o->ancestor = "merged common ancestors";
 	clean = merge_trees(o, h1->tree, h2->tree, merged_common_ancestors->tree,
 			    &mrtree);
 
diff --git a/merge-recursive.h b/merge-recursive.h
index be8410ad18..d1192f56d7 100644
--- a/merge-recursive.h
+++ b/merge-recursive.h
@@ -4,6 +4,7 @@
 #include "string-list.h"
 
 struct merge_options {
+	const char *ancestor;
 	const char *branch1;
 	const char *branch2;
 	enum {
diff --git a/rerere.c b/rerere.c
index a59f74f76c..f221bed1e9 100644
--- a/rerere.c
+++ b/rerere.c
@@ -319,7 +319,7 @@ static int handle_cache(const char *path, unsigned char *sha1, const char *outpu
 		if (!mmfile[i].ptr && !mmfile[i].size)
 			mmfile[i].ptr = xstrdup("");
 	}
-	ll_merge(&result, path, &mmfile[0],
+	ll_merge(&result, path, &mmfile[0], NULL,
 		 &mmfile[1], "ours",
 		 &mmfile[2], "theirs", 0);
 	for (i = 0; i < 3; i++)
@@ -376,7 +376,7 @@ static int merge(const char *name, const char *path)
 		ret = 1;
 		goto out;
 	}
-	ret = ll_merge(&result, path, &base, &cur, "", &other, "", 0);
+	ret = ll_merge(&result, path, &base, NULL, &cur, "", &other, "", 0);
 	if (!ret) {
 		FILE *f = fopen(path, "w");
 		if (!f)
diff --git a/t/t3507-cherry-pick-conflict.sh b/t/t3507-cherry-pick-conflict.sh
new file mode 100644
index 0000000000..e25cf8039a
--- /dev/null
+++ b/t/t3507-cherry-pick-conflict.sh
@@ -0,0 +1,198 @@
+#!/bin/sh
+
+test_description='test cherry-pick and revert with conflicts
+
+  -
+  + picked: rewrites foo to c
+  + base: rewrites foo to b
+  + initial: writes foo as a, unrelated as unrelated
+
+'
+
+. ./test-lib.sh
+
+test_expect_success setup '
+
+	echo unrelated >unrelated &&
+	git add unrelated &&
+	test_commit initial foo a &&
+	test_commit base foo b &&
+	test_commit picked foo c &&
+	git config advice.detachedhead false
+
+'
+
+test_expect_success 'failed cherry-pick does not advance HEAD' '
+
+	git checkout -f initial^0 &&
+	git read-tree -u --reset HEAD &&
+	git clean -d -f -f -q -x &&
+
+	git update-index --refresh &&
+	git diff-index --exit-code HEAD &&
+
+	head=$(git rev-parse HEAD) &&
+	test_must_fail git cherry-pick picked &&
+	newhead=$(git rev-parse HEAD) &&
+
+	test "$head" = "$newhead"
+'
+
+test_expect_success 'failed cherry-pick produces dirty index' '
+
+	git checkout -f initial^0 &&
+	git read-tree -u --reset HEAD &&
+	git clean -d -f -f -q -x &&
+
+	git update-index --refresh &&
+	git diff-index --exit-code HEAD &&
+
+	test_must_fail git cherry-pick picked &&
+
+	test_must_fail git update-index --refresh -q &&
+	test_must_fail git diff-index --exit-code HEAD
+'
+
+test_expect_success 'failed cherry-pick registers participants in index' '
+
+	git read-tree -u --reset HEAD &&
+	git clean -d -f -f -q -x &&
+	{
+		git checkout base -- foo &&
+		git ls-files --stage foo &&
+		git checkout initial -- foo &&
+		git ls-files --stage foo &&
+		git checkout picked -- foo &&
+		git ls-files --stage foo
+	} > stages &&
+	sed "
+		1 s/ 0	/ 1	/
+		2 s/ 0	/ 2	/
+		3 s/ 0	/ 3	/
+	" < stages > expected &&
+	git checkout -f initial^0 &&
+
+	git update-index --refresh &&
+	git diff-index --exit-code HEAD &&
+
+	test_must_fail git cherry-pick picked &&
+	git ls-files --stage --unmerged > actual &&
+
+	test_cmp expected actual
+'
+
+test_expect_success 'failed cherry-pick describes conflict in work tree' '
+
+	git checkout -f initial^0 &&
+	git read-tree -u --reset HEAD &&
+	git clean -d -f -f -q -x &&
+	cat <<-EOF > expected &&
+	<<<<<<< HEAD
+	a
+	=======
+	c
+	>>>>>>> objid picked
+	EOF
+
+	git update-index --refresh &&
+	git diff-index --exit-code HEAD &&
+
+	test_must_fail git cherry-pick picked &&
+
+	sed "s/[a-f0-9]*\.\.\./objid/" foo > actual &&
+	test_cmp expected actual
+'
+
+test_expect_success 'diff3 -m style' '
+
+	git config merge.conflictstyle diff3 &&
+	git checkout -f initial^0 &&
+	git read-tree -u --reset HEAD &&
+	git clean -d -f -f -q -x &&
+	cat <<-EOF > expected &&
+	<<<<<<< HEAD
+	a
+	||||||| parent of objid picked
+	b
+	=======
+	c
+	>>>>>>> objid picked
+	EOF
+
+	git update-index --refresh &&
+	git diff-index --exit-code HEAD &&
+
+	test_must_fail git cherry-pick picked &&
+
+	sed "s/[a-f0-9]*\.\.\./objid/" foo > actual &&
+	test_cmp expected actual
+'
+
+test_expect_success 'revert also handles conflicts sanely' '
+
+	git config --unset merge.conflictstyle &&
+	git read-tree -u --reset HEAD &&
+	git clean -d -f -f -q -x &&
+	cat <<-EOF > expected &&
+	<<<<<<< HEAD
+	a
+	=======
+	b
+	>>>>>>> parent of objid picked
+	EOF
+	{
+		git checkout picked -- foo &&
+		git ls-files --stage foo &&
+		git checkout initial -- foo &&
+		git ls-files --stage foo &&
+		git checkout base -- foo &&
+		git ls-files --stage foo
+	} > stages &&
+	sed "
+		1 s/ 0	/ 1	/
+		2 s/ 0	/ 2	/
+		3 s/ 0	/ 3	/
+	" < stages > expected-stages &&
+	git checkout -f initial^0 &&
+
+	git update-index --refresh &&
+	git diff-index --exit-code HEAD &&
+
+	head=$(git rev-parse HEAD) &&
+	test_must_fail git revert picked &&
+	newhead=$(git rev-parse HEAD) &&
+	git ls-files --stage --unmerged > actual-stages &&
+
+	test "$head" = "$newhead" &&
+	test_must_fail git update-index --refresh -q &&
+	test_must_fail git diff-index --exit-code HEAD &&
+	test_cmp expected-stages actual-stages &&
+	sed "s/[a-f0-9]*\.\.\./objid/" foo > actual &&
+	test_cmp expected actual
+'
+
+test_expect_success 'revert conflict, diff3 -m style' '
+	git config merge.conflictstyle diff3 &&
+	git checkout -f initial^0 &&
+	git read-tree -u --reset HEAD &&
+	git clean -d -f -f -q -x &&
+	cat <<-EOF > expected &&
+	<<<<<<< HEAD
+	a
+	||||||| objid picked
+	c
+	=======
+	b
+	>>>>>>> parent of objid picked
+	EOF
+
+	git update-index --refresh &&
+	git diff-index --exit-code HEAD &&
+
+	test_must_fail git revert picked &&
+
+	sed "s/[a-f0-9]*\.\.\./objid/" foo > actual &&
+	test_cmp expected actual
+'
+
+test_done
diff --git a/t/t6023-merge-file.sh b/t/t6023-merge-file.sh
index 5034dd1352..d486d73994 100755
--- a/t/t6023-merge-file.sh
+++ b/t/t6023-merge-file.sh
@@ -181,7 +181,7 @@ et nihil mihi deerit;
 
 In loco pascuae ibi me collocavit;
 super aquam refectionis educavit me.
-|||||||
+||||||| new5.txt
 et nihil mihi deerit.
 In loco pascuae ibi me collocavit,
 super aquam refectionis educavit me;
@@ -225,7 +225,7 @@ et nihil mihi deerit;
 
 In loco pascuae ibi me collocavit;
 super aquam refectionis educavit me.
-||||||||||
+|||||||||| new5.txt
 et nihil mihi deerit.
 In loco pascuae ibi me collocavit,
 super aquam refectionis educavit me;
diff --git a/t/t7201-co.sh b/t/t7201-co.sh
index d20ed61b48..1337fa5a22 100755
--- a/t/t7201-co.sh
+++ b/t/t7201-co.sh
@@ -11,10 +11,12 @@ Test switching across them.
   ! [master] Initial A one, A two
    * [renamer] Renamer R one->uno, M two
     ! [side] Side M one, D two, A three
-  ---
-    + [side] Side M one, D two, A three
-   *  [renamer] Renamer R one->uno, M two
-  +*+ [master] Initial A one, A two
+     ! [simple] Simple D one, M two
+  ----
+     + [simple] Simple D one, M two
+    +  [side] Side M one, D two, A three
+   *   [renamer] Renamer R one->uno, M two
+  +*++ [master] Initial A one, A two
 
 '
 
@@ -52,6 +54,11 @@ test_expect_success setup '
 	git update-index --add --remove one two three &&
 	git commit -m "Side M one, D two, A three" &&
 
+	git checkout -b simple master &&
+	rm -f one &&
+	fill a c e > two &&
+	git commit -a -m "Simple D one, M two" &&
+
 	git checkout master
 '
 
@@ -166,6 +173,56 @@ test_expect_success 'checkout -m with merge conflict' '
 	! test -s current
 '
 
+test_expect_success 'format of merge conflict from checkout -m' '
+
+	git checkout -f master && git clean -f &&
+
+	fill b d > two &&
+	git checkout -m simple &&
+
+	git ls-files >current &&
+	fill same two two two >expect &&
+	test_cmp current expect &&
+
+	cat <<-EOF >expect &&
+	<<<<<<< simple
+	a
+	c
+	e
+	=======
+	b
+	d
+	>>>>>>> local
+	EOF
+	test_cmp two expect
+'
+
+test_expect_success 'checkout --merge --conflict=diff3 <branch>' '
+
+	git checkout -f master && git reset --hard && git clean -f &&
+
+	fill b d > two &&
+	git checkout --merge --conflict=diff3 simple &&
+
+	cat <<-EOF >expect &&
+	<<<<<<< simple
+	a
+	c
+	e
+	||||||| master
+	a
+	b
+	c
+	d
+	e
+	=======
+	b
+	d
+	>>>>>>> local
+	EOF
+	test_cmp two expect
+'
+
 test_expect_success 'checkout to detach HEAD (with advice declined)' '
 
 	git config advice.detachedHead false &&
@@ -481,7 +538,7 @@ test_expect_success 'checkout with --merge, in diff3 -m style' '
 	(
 		echo "<<<<<<< ours"
 		echo ourside
-		echo "|||||||"
+		echo "||||||| base"
 		echo original
 		echo "======="
 		echo theirside
@@ -525,7 +582,7 @@ test_expect_success 'checkout --conflict=diff3' '
 	(
 		echo "<<<<<<< ours"
 		echo ourside
-		echo "|||||||"
+		echo "||||||| base"
 		echo original
 		echo "======="
 		echo theirside
diff --git a/xdiff/xdiff.h b/xdiff/xdiff.h
index a71763ade8..711048ea36 100644
--- a/xdiff/xdiff.h
+++ b/xdiff/xdiff.h
@@ -117,12 +117,14 @@ typedef struct s_xmparam {
 	int level;
 	int favor;
 	int style;
+	const char *ancestor;	/* label for orig */
+	const char *file1;	/* label for mf1 */
+	const char *file2;	/* label for mf2 */
 } xmparam_t;
 
 #define DEFAULT_CONFLICT_MARKER_SIZE 7
 
-int xdl_merge(mmfile_t *orig, mmfile_t *mf1, const char *name1,
-		mmfile_t *mf2, const char *name2,
+int xdl_merge(mmfile_t *orig, mmfile_t *mf1, mmfile_t *mf2,
 		xmparam_t const *xmp, mmbuffer_t *result);
 
 #ifdef __cplusplus
diff --git a/xdiff/xmerge.c b/xdiff/xmerge.c
index 87cafa7021..16dd9acd37 100644
--- a/xdiff/xmerge.c
+++ b/xdiff/xmerge.c
@@ -145,11 +145,13 @@ static int xdl_orig_copy(xdfenv_t *xe, int i, int count, int add_nl, char *dest)
 
 static int fill_conflict_hunk(xdfenv_t *xe1, const char *name1,
 			      xdfenv_t *xe2, const char *name2,
+			      const char *name3,
 			      int size, int i, int style,
 			      xdmerge_t *m, char *dest, int marker_size)
 {
 	int marker1_size = (name1 ? strlen(name1) + 1 : 0);
 	int marker2_size = (name2 ? strlen(name2) + 1 : 0);
+	int marker3_size = (name3 ? strlen(name3) + 1 : 0);
 	int j;
 
 	if (marker_size <= 0)
@@ -179,10 +181,15 @@ static int fill_conflict_hunk(xdfenv_t *xe1, const char *name1,
 	if (style == XDL_MERGE_DIFF3) {
 		/* Shared preimage */
 		if (!dest) {
-			size += marker_size + 1;
+			size += marker_size + 1 + marker3_size;
 		} else {
 			for (j = 0; j < marker_size; j++)
 				dest[size++] = '|';
+			if (marker3_size) {
+				dest[size] = ' ';
+				memcpy(dest + size + 1, name3, marker3_size - 1);
+				size += marker3_size;
+			}
 			dest[size++] = '\n';
 		}
 		size += xdl_orig_copy(xe1, m->i0, m->chg0, 1,
@@ -217,6 +224,7 @@ static int fill_conflict_hunk(xdfenv_t *xe1, const char *name1,
 
 static int xdl_fill_merge_buffer(xdfenv_t *xe1, const char *name1,
 				 xdfenv_t *xe2, const char *name2,
+				 const char *ancestor_name,
 				 int favor,
 				 xdmerge_t *m, char *dest, int style,
 				 int marker_size)
@@ -229,6 +237,7 @@ static int xdl_fill_merge_buffer(xdfenv_t *xe1, const char *name1,
 
 		if (m->mode == 0)
 			size = fill_conflict_hunk(xe1, name1, xe2, name2,
+						  ancestor_name,
 						  size, i, style, m, dest,
 						  marker_size);
 		else if (m->mode & 3) {
@@ -398,11 +407,15 @@ static int xdl_simplify_non_conflicts(xdfenv_t *xe1, xdmerge_t *m,
  *
  * returns < 0 on error, == 0 for no conflicts, else number of conflicts
  */
-static int xdl_do_merge(xdfenv_t *xe1, xdchange_t *xscr1, const char *name1,
-		xdfenv_t *xe2, xdchange_t *xscr2, const char *name2,
-		xmparam_t const *xmp, mmbuffer_t *result) {
+static int xdl_do_merge(xdfenv_t *xe1, xdchange_t *xscr1,
+		xdfenv_t *xe2, xdchange_t *xscr2,
+		xmparam_t const *xmp, mmbuffer_t *result)
+{
 	xdmerge_t *changes, *c;
 	xpparam_t const *xpp = &xmp->xpp;
+	const char *const ancestor_name = xmp->ancestor;
+	const char *const name1 = xmp->file1;
+	const char *const name2 = xmp->file2;
 	int i0, i1, i2, chg0, chg1, chg2;
 	int level = xmp->level;
 	int style = xmp->style;
@@ -540,6 +553,7 @@ static int xdl_do_merge(xdfenv_t *xe1, xdchange_t *xscr1, const char *name1,
 	if (result) {
 		int marker_size = xmp->marker_size;
 		int size = xdl_fill_merge_buffer(xe1, name1, xe2, name2,
+						 ancestor_name,
 						 favor, changes, NULL, style,
 						 marker_size);
 		result->ptr = xdl_malloc(size);
@@ -548,15 +562,16 @@ static int xdl_do_merge(xdfenv_t *xe1, xdchange_t *xscr1, const char *name1,
 			return -1;
 		}
 		result->size = size;
-		xdl_fill_merge_buffer(xe1, name1, xe2, name2, favor, changes,
+		xdl_fill_merge_buffer(xe1, name1, xe2, name2,
+				      ancestor_name, favor, changes,
 				      result->ptr, style, marker_size);
 	}
 	return xdl_cleanup_merge(changes);
 }
 
-int xdl_merge(mmfile_t *orig, mmfile_t *mf1, const char *name1,
-		mmfile_t *mf2, const char *name2,
-		xmparam_t const *xmp, mmbuffer_t *result) {
+int xdl_merge(mmfile_t *orig, mmfile_t *mf1, mmfile_t *mf2,
+		xmparam_t const *xmp, mmbuffer_t *result)
+{
 	xdchange_t *xscr1, *xscr2;
 	xdfenv_t xe1, xe2;
 	int status;
@@ -591,8 +606,8 @@ int xdl_merge(mmfile_t *orig, mmfile_t *mf1, const char *name1,
 		memcpy(result->ptr, mf1->ptr, mf1->size);
 		result->size = mf1->size;
 	} else {
-		status = xdl_do_merge(&xe1, xscr1, name1,
-				      &xe2, xscr2, name2,
+		status = xdl_do_merge(&xe1, xscr1,
+				      &xe2, xscr2,
 				      xmp, result);
 	}
 	xdl_free_script(xscr1);