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