git-p4: support excluding paths on sync

git-p4: support excluding paths on sync

The clone subcommand has long had support for excluding
subdirectories, but sync has not. This is a nuisance,
since as soon as you do a sync, any changed files that
were initially excluded start showing up.

Move the "exclude" command-line option into the parent
class; the actual behavior was already present there so
it simply had to be exposed.

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