Fix race condition inside `git diff-files`
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

uber/arcanist,Symplicity/arcanist,coursera/arcanist,Symplicity/arcanist,zhihu/arcanist,wikimedia/phabricator-arcanist,hach-que/arcanist,eSpark/arcanist,jra3/arcanist,fivestars/arcanist,hach-que/arcanist,codelabs-ch/arcanist,PageUpPeopleOrg/arcanist-patches,Symplicity/arcanist,jra3/arcanist,calblueprint/arcanist,fivestars/arcanist,r4nt/arcanist,fivestars/arcanist,kevinseelbach/arcanist,coursera/arcanist,dguo-coursera/arcanist,haskell-infra/arcanist,chundongwang/arcanist,PageUpPeopleOrg/arcanist-patches,chundongwang/arcanist,r4nt/arcanist,chundongwang/arcanist,wharris/arcanist,codelabs-ch/arcanist,coursera/arcanist,calblueprint/arcanist,alexmv/arcanist,KieranHayes/arcanist,uber/arcanist,dguo-coursera/arcanist,sneakpeeq/arcanist,phacility/arcanist,kevinseelbach/arcanist,uber/arcanist,alexmv/arcanist,folsom-labs/arcanist,phacility/arcanist,wharris/arcanist,memsql/arcanist,eSpark/arcanist,aik099/arcanist,KieranHayes/arcanist,folsom-labs/arcanist,folsom-labs/arcanist,calblueprint/arcanist,wikimedia/phabricator-arcanist,dguo-coursera/arcanist,zhihu/arcanist,wxstars/arcanist,KieranHayes/arcanist,r4nt/arcanist,r4nt/arcanist,Khan/arcanist,fivestars/arcanist,hach-que/arcanist,wharris/arcanist,memsql/arcanist,KieranHayes/arcanist,phacility/arcanist,Khan/arcanist,aik099/arcanist,codelabs-ch/arcanist,hach-que/arcanist,PageUpPeopleOrg/arcanist-patches,alexmv/arcanist,phacility/arcanist,kevinseelbach/arcanist,calblueprint/arcanist,haskell-infra/arcanist,eSpark/arcanist,wikimedia/phabricator-arcanist,kevinseelbach/arcanist,phacility/arcanist,memsql/arcanist,codelabs-ch/arcanist,sneakpeeq/arcanist,memsql/arcanist,coursera/arcanist,uber/arcanist,wharris/arcanist,zhihu/arcanist,wikimedia/phabricator-arcanist,aik099/arcanist,wxstars/arcanist,eSpark/arcanist,sneakpeeq/arcanist,codelabs-ch/arcanist,zhihu/arcanist,folsom-labs/arcanist,Khan/arcanist,Khan/arcanist,Symplicity/arcanist,wxstars/arcanist,dguo-coursera/arcanist,jra3/arcanist,chundongwang/arcanist,haskell-infra/arcanist,haskell-infra/arcanist,sneakpeeq/arcanist,wxstars/arcanist,PageUpPeopleOrg/arcanist-patches,alexmv/arcanist,aik099/arcanist,jra3/arcanist