git p4: Use git diff-tree instead of format-patch

The output of git format-patch can vary with user preferences. In
particular setting diff.noprefix will break the "git apply" that
is done as part of "git p4 submit".

Acked-by: Pete Wyckoff <pw@padd.com>
Signed-off-by: Crestez Dan Leonard <cdleonard@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Merge branch 'cl/p4-use-diff-tree' into pu

* cl/p4-use-diff-tree:
  git p4: Use git diff-tree instead of format-patch