git-p4: add option to disable syncing of p4/master with p4

git-p4: add option to disable syncing of p4/master with p4

Add an option to the git-p4 submit command to disable syncing
with Perforce.

This is useful for the case where a git-p4 mirror has been setup
on a server somewhere, running from (e.g.) cron, and developers
then clone from this. Having the local cloned copy also sync
from Perforce just isn't useful.

Signed-off-by: Luke Diamand <luke@diamand.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>