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>
Nguyễn Thái Ngọc Duy