Add branch as an option to recentcommitsbypath API

Summary: We need to be able to request history for more than the master branch (the default). This adds branch as an option to the API.

Test Plan: Test by sending recentcommitsbypath a non-master branch along with callsign.

Reviewers: epriestley, chad, btrahan

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5967

src/applications/diffusion/conduit/ConduitAPI_diffusion_getrecentcommitsbypath_Method.php

···
     return array(
       'callsign' => 'required string',
       'path' => 'required string',
+      'branch' => 'optional string',
       'limit' => 'optional int',
     );
   }
···
         'user' => $request->getUser(),
         'callsign' => $request->getValue('callsign'),
         'path' => $request->getValue('path'),
+        'branch' => $request->getValue('branch'),
       ));
 
     $limit = nonempty(