git p4: avoid shell when invoking git rev-list
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 <1a91d62f7ca67399625a4368a6ab5d4a3baa6073@padd.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