Accept and route VCS HTTP requests

Summary:
Mostly ripped from D7391, with some changes:

  - Serve repositories at `/diffusion/X/`, with no special `/git/` or `/serve/` URI component.
    - This requires a little bit of magic, but I got the magic working for Git, Mercurial and SVN, and it seems reasonable.
    - I think having one URI for everything will make it easier for users to understand.
    - One downside is that git will clone into `X` by default, but I think that's not a big deal, and we can work around that in the future easily enough.
  - Accept HTTP requests for Git, SVN and Mercurial repositories.
  - Auth logic is a little different in order to be more consistent with how other things work.
  - Instead of AphrontBasicAuthResponse, added "VCSResponse". Mercurial can print strings we send it on the CLI if we're careful, so support that. I did a fair amount of digging and didn't have any luck with git or svn.
  - Commands we don't know about are assumed to require "Push" capability by default.

No actual VCS data going over the wire yet.

Test Plan:
Ran a bunch of stuff like this:

  $ hg clone http://local.aphront.com:8080/diffusion/P/
  abort: HTTP Error 403: This repository is not available over HTTP.

...and got pretty reasonable-seeming errors in all cases. All this can do is produce errors for now.

Reviewers: hach-que, btrahan

Reviewed By: hach-que

CC: aran

Maniphest Tasks: T2230

Differential Revision: https://secure.phabricator.com/D7417

src/applications/diffusion/controller/DiffusionRepositoryDefaultController.php

@@ -0,0 +1,11 @@
+<?php
+
+final class DiffusionRepositoryDefaultController extends DiffusionController {
+
+  public function processRequest() {
+    // NOTE: This controller is just here to make sure we call
+    // willBeginExecution() on any /diffusion/X/ URI, so we can intercept
+    // `git`, `hg` and `svn` HTTP protocol requests.
+    return new Aphront404Response();
+  }
+}