Optimize mobile layout of DiffusionHistoryView

Summary: Little nits and spacing changes to viewing diffusion commit history on phones.

Test Plan:
Review in Chrome, iOS Simulator.

{F4990749}

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D18085

src/applications/diffusion/controller/DiffusionHistoryController.php

@@ -69,7 +69,8 @@ final class DiffusionHistoryController extends DiffusionController {
     return $this->newPage()
       ->setTitle($title)
       ->setCrumbs($crumbs)
-      ->appendChild($view);
+      ->appendChild($view)
+      ->addClass('diffusion-history-view');
   }
 
   private function buildHeader(DiffusionRequest $drequest) {