merge: record tag objects without peeling in MERGE_HEAD

Otherwise, "git commit" wouldn't have a way to tell that we were in the
middle of merging an annotated or signed tag, not a plain commit, after
"git merge" stops to ask the user to resolve conflicts.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/merge.c

@@ -1045,9 +1045,16 @@ static void write_merge_state(void)
 	struct commit_list *j;
 	struct strbuf buf = STRBUF_INIT;
 
-	for (j = remoteheads; j; j = j->next)
+	for (j = remoteheads; j; j = j->next) {
-		strbuf_addf(&buf, "%s\n",
+		unsigned const char *sha1;
-			sha1_to_hex(j->item->object.sha1));
+		struct commit *c = j->item;
+		if (c->util && merge_remote_util(c)->obj) {
+			sha1 = merge_remote_util(c)->obj->sha1;
+		} else {
+			sha1 = c->object.sha1;
+		}
+		strbuf_addf(&buf, "%s\n", sha1_to_hex(sha1));
+	}
 	fd = open(git_path("MERGE_HEAD"), O_WRONLY | O_CREAT, 0666);
 	if (fd < 0)
 		die_errno(_("Could not open '%s' for writing"),

t/t7600-merge.sh

@@ -96,7 +96,11 @@ verify_parents () {
 
 verify_mergeheads () {
 	printf '%s\n' "$@" >mergehead.expected &&
-	test_cmp mergehead.expected .git/MERGE_HEAD
+	while read sha1 rest
+	do
+		git rev-parse $sha1
+	done <.git/MERGE_HEAD >mergehead.actual &&
+	test_cmp mergehead.expected mergehead.actual
 }
 
 verify_no_mergehead () {