Compare two branches

Summary:
This shows the commits list only (Actual `git diff` will show up at a later date).
The inputs are left as text-fields, to allow the form to accept anything that can be resolved. The form is GET, to allow sharing URIs.

The conduit method response array is compatible with that of `diffusion.historyquery`, to make it easy to build
the "history" table.

The hardest part here was, of course, Naming. I think "from" and "onto" are unconfusing, and I'm fairly confident that the "to merge"
instructions are in sync with the actual content of the page.

Test Plan: Look at several "compare" views, with various values of "from" and "onto".

Reviewers: #blessed_reviewers!, epriestley

Subscribers: caov297, 20after4, Sam2304, reardencode, baileyb, chad, Korvin

Maniphest Tasks: T929

Differential Revision: https://secure.phabricator.com/D15330

src/applications/diffusion/controller/DiffusionController.php

@@ -213,6 +213,9 @@ abstract class DiffusionController extends PhabricatorController {
       case 'change':
         $view_name = pht('Change');
         break;
+      case 'compare':
+        $view_name = pht('Compare');
+        break;
     }
 
     $crumb = id(new PHUICrumbView())