Restore kept branch after landing

Test Plan: Ran it separately.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4846
Upload binary files diffs in parallel

Summary: Makes sense after D2471.

Test Plan:
Swapped two binary files, ran `arc diff --only`.
Saw time 3.797 s instead of 4.361 s.

Changed `file.upload` to `file.uploa`, saw proper error.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4847