Redirect `/source/x` (no slash) to `/source/x/` (canonical) when viewer is logged out and "x" is public

Summary:
Fixes T12035. Normally, the "abc" -> "abc/" redirect is handled automatically when "abc" hits a 404.

However, in this case, "source/x" does not 404. We route this to a valid controller because some VCS requests omit the slashes, then manually perform the redirect if we aren't serving a VCS request.

Allow this controller to serve public resources so we can serve the redirect to logged-out users instead of prompting them to login so they can be redirected.

Test Plan: Visited `/source/x` as a logged-out user, where `x` is a public repository.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12035

Differential Revision: https://secure.phabricator.com/D17097

src/applications/diffusion/controller/DiffusionRepositoryDefaultController.php

···
 
 final class DiffusionRepositoryDefaultController extends DiffusionController {
 
+  public function shouldAllowPublic() {
+    // NOTE: We allow public access to this controller because it handles
+    // redirecting paths that are missing a trailing "/". We need to manually
+    // redirect these instead of relying on the automatic redirect because
+    // some VCS requests may omit the slashes. See T12035, and below, for some
+    // discussion.
+    return true;
+  }
+
   public function handleRequest(AphrontRequest $request) {
     $response = $this->loadDiffusionContext();
     if ($response) {