speed up _get_file_diff slightly

This func just wants to scan & extract the diff, so including the commit
message is pointless.  Tweak the git show command so that it doesn't even
bother omitting that.

TEST=`./pre-upload_unittest.py` passes
TEST=ran against some CLs w/bad diffs and made sure they were still caught

Change-Id: I018ad730f2cad627fd18baa257b8437ba1d43e8c
Reviewed-on: https://chromium-review.googlesource.com/184610
Reviewed-by: David James <davidjames@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
1 file changed