Adding the diff.noprefix flag to .gitconfig broke python
formatting because git diffs were being parsed with regex
that relied on the default git diff prefixs.

This CL fix's this issue by always specifying prefix flags
for git diffs when formatting python.

Bug:846432
Change-Id: Ifde305da9574e6b46455a0237703b7364fac77e4
Reviewed-on: https://chromium-review.googlesource.com/c/1349809Reviewed-by: Marc-Antoine Ruel <maruel@chromium.org>
Commit-Queue: Marc-Antoine Ruel <maruel@chromium.org>