R1:9a2e45ef12ea

Serve Git reads over SSH

Summary: Like D7423, but for SSH.

Test Plan: Ran `git clone ssh://...`, got a clone.

Reviewers: btrahan

Reviewed By: btrahan

CC: hach-que, aran

Maniphest Tasks: T2230

Differential Revision: https://secure.phabricator.com/D7424