Support ID-based repository URIs, and canonicalize repository URIs

Summary:
Ref T4245. Make `/diffusion/123/` work, but redirect the user to `/diffusion/XYZ/` if the repository has a callsign.

(Right now, every repository has a callsign, so this always redirects.)

Also redirect `/R123:abcdef` if the repository has a callsign.

Also also, move the Pull garbage collector somewhere more sensible.

Test Plan:
  - Added test coverage.
  - Visited `/diffusion/1/`, was redirected.
  - Visited `/diffusion/R1:abcdef`, was redirected.
  - Browsed Diffusion normally.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4245

Differential Revision: https://secure.phabricator.com/D15301

src/applications/diffusion/controller/DiffusionController.php

@@ -64,6 +64,20 @@ abstract class DiffusionController extends PhabricatorController {
       return new Aphront404Response();
     }
 
+    // If the client is making a request like "/diffusion/1/...", but the
+    // repository has a different canonical path like "/diffusion/XYZ/...",
+    // redirect them to the canonical path.
+
+    $request_path = $request->getPath();
+    $repository = $drequest->getRepository();
+
+    $canonical_path = $repository->getCanonicalPath($request_path);
+    if ($canonical_path !== null) {
+      if ($canonical_path != $request_path) {
+        return id(new AphrontRedirectResponse())->setURI($canonical_path);
+      }
+    }
+
     $this->diffusionRequest = $drequest;
 
     return null;