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

scripts/ssh/ssh-exec.php

@@ -63,6 +63,7 @@ try {
     new ConduitSSHWorkflow(),
 
     new DiffusionSSHGitUploadPackWorkflow(),
+    new DiffusionSSHGitReceivePackWorkflow(),
   );
 
   $workflow_names = mpull($workflows, 'getName', 'getName');