Since d1c5f2a (Add git-am, applymbox replacement., 2005-10-07),
git-am.sh will invoke the post-applypatch hook after the patch is
applied and a commit is made. The exit code of the hook is ignored.
Re-implement this in builtin/am.c.
Signed-off-by: Paul Tan <pyokagan@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>