Split Diffusion VSC serve code into its own controller

Summary: This is starting to get a bit sizable and it turns out Mercurial is sort of a beast, so split the VCS serve stuff into a separate controller.

Test Plan: Pushed and pulled an authenticated Git repository.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, hach-que

Differential Revision: https://secure.phabricator.com/D7494