Add more options to git-clang-format.py

This CL adds:
  1. --diff to show the clang-format diff output.
  2. HEAD as default value for --commit option.

BUG=b:30824236
TEST=repo upload

Change-Id: I7fc14f501649ec8ab0bdcbe1950ffcbb8303547a
Reviewed-on: https://chromium-review.googlesource.com/412680
Commit-Ready: Ricky Liang <jcliang@chromium.org>
Tested-by: Ricky Liang <jcliang@chromium.org>
Reviewed-by: Wu-cheng Li <wuchengli@chromium.org>
1 file changed