R1:7d9dfb561dd5

Serve Git reads over HTTP

Summary: Mostly ripped from D7391. No writes yet.

Test Plan: Ran `git clone` against a local over HTTP, got a clone.

Reviewers: btrahan, hach-que

Reviewed By: hach-que

CC: aran

Maniphest Tasks: T2230

Differential Revision: https://secure.phabricator.com/D7423