Accept tags in HEAD or MERGE_HEAD

HEAD and MERGE_HEAD (among other branch tips) should never hold a
tag. That can only be caused by broken tools and is cumbersome to fix
by an end user with:

  $ git update-ref HEAD $(git rev-parse HEAD^{commit})

which may look like a magic to a new person.

Be easy, warn users (so broken tools can be fixed if they bother to
report) and move on.

Be robust, if the given SHA-1 cannot be resolved to a commit object,
die (therefore return value is always valid).

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/builtin/fmt-merge-msg.c b/builtin/fmt-merge-msg.c
index 75816329d6..7e2f22589d 100644
--- a/builtin/fmt-merge-msg.c
+++ b/builtin/fmt-merge-msg.c
@@ -293,7 +293,7 @@ static int do_fmt_merge_msg(int merge_title, struct strbuf *in,
 		struct commit *head;
 		struct rev_info rev;
 
-		head = lookup_commit(head_sha1);
+		head = lookup_commit_or_die(head_sha1, "HEAD");
 		init_revisions(&rev, NULL);
 		rev.commit_format = CMIT_FMT_ONELINE;
 		rev.ignore_merges = 1;