Implement writes over HTTP for Git.

Summary: Depends on D7642.  This updates the authentication logic so that HTTP writes can be made to Git repositories hosted by Phabricator.

Test Plan: Set the policy to allow me to push and I was able to.  Changed the policy to disallow push and I was no longer able to push.

Reviewers: #blessed_reviewers, hach-que

Reviewed By: hach-que

CC: Korvin, epriestley, aran

Maniphest Tasks: T2230

Differential Revision: https://secure.phabricator.com/D7468