git p4: avoid shell when invoking git rev-list

Invoke git rev-list directly, avoiding the shell, in
P4Submit and P4Sync.  The overhead of starting extra
processes is significant in cygwin; this speeds things
up on that platform.

Signed-off-by: Pete Wyckoff <pw@padd.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>