Land Revision button for hosted git repos

Summary:
ref T182.

Simple approach of clone, patch, push. While waiting for drydock, implement a hackish mutex
setup for the workspace, which should work ok as long as there's only one committer who is
carefull about theses things.

Less obvious note: This is taking the both author and commiter's 'primary email' for the commit -
which might rub some people wrong.

Test Plan:
With a hosted repo, created some diffs and landed them.
Also clicked button for some error cases, got the right error message.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: hach-que, Korvin, epriestley, aran

Maniphest Tasks: T182

Differential Revision: https://secure.phabricator.com/D7486