builtin-am: implement -s/--signoff
builtin-am: implement -s/--signoff

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 <ca5433e1cdd25b4a4001c54525dea593b33edc0c@gmail.com>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git