Notify.pm: reduce mail.sh spew
Do not send mail.sh any refs it will complain about.
It only sends messages for refs/heads/* and refs/tags/*, so
don't send it anything else.
Also don't send it refs/heads/* refs unless notifymail is
set whereas refs/tags/* are sent if either notifymail or
notifytag is set.
This is not perfect as a non-annotated tag in refs/tag/* will
still be sent when notifytag is set but notifymail is not
and mail.sh will complain about that.
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>