Fix race condition inside git diff-files

Summary:
There's some sort of race inside git here, where the git diff-files command exits with different results some of the time when run in parallel with git ls-files or git diff (running either command was sufficient to trigger the race).

Run it separately to avoid the race.

I poked around the git source a little bit but quickly lost interest given that the issue seems fixed and this workaround is essentially reasonable.

Test Plan: Ran test 20x in a row without failures.

Reviewers: hach-que

Reviewed By: hach-que

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D9616