Serve git writes over SSH

Summary: Looks like this is pretty straightforward; same as the reads except mark it as needing PUSH.

Test Plan: Ran git push, pushed over SSH to a hosted repo.

Reviewers: btrahan

Reviewed By: btrahan

CC: hach-que, aran

Maniphest Tasks: T2230

Differential Revision: https://secure.phabricator.com/D7425