fail arc diff if second lfs push errors

Summary:
We are having issues where people run out of file descriptors and the first git push will succeed, but the
second one will not. We'd like the diff to not be created in this case as it leads to weird behavior like our tests
running against 0 changed files.

Test Plan: none

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D21471