Modernize more Diffusion controllers

Summary: Ref T4245. Standardize how context is read, minor updates / modernizations / consistency tweaks.

Test Plan:
  - Viewed a change.
  - Viewed brnaches.
  - Edited a commit.
  - Viewed tags.
  - Viewed history.
  - Added, edited and deleted a mirror.
  - Viewed push events.
  - Viewed a particular event.
  - Viewed ref disambiguation.
  - Viewed repository list.
  - Ran automation test.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4245

Differential Revision: https://secure.phabricator.com/D14944

src/applications/diffusion/controller/DiffusionRepositoryTestAutomationController.php

@@ -3,9 +3,14 @@
 final class DiffusionRepositoryTestAutomationController
   extends DiffusionRepositoryEditController {
 
-  protected function processDiffusionRequest(AphrontRequest $request) {
+  public function handleRequest(AphrontRequest $request) {
+    $response = $this->loadDiffusionContext();
+    if ($response) {
+      return $response;
+    }
+
     $viewer = $this->getViewer();
-    $drequest = $this->diffusionRequest;
+    $drequest = $this->getDiffusionRequest();
     $repository = $drequest->getRepository();
 
     $repository = id(new PhabricatorRepositoryQuery())