Since d1c5f2a (Add git-am, applymbox replacement., 2005-10-07), git-am
supported the --signoff option which will append a signoff at the end of
the commit messsage. Re-implement this feature in parse_mail() by
calling append_signoff() if the option is set.
Signed-off-by: Paul Tan <pyokagan@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>