Continue reducing callers to getCallsign()

Summary: Ref T4245. More of the same, just narrowing down the easy cases.

Test Plan:
- Called `diffusion.querycommit`.
- Browsed branches.
- Browsed repository.
- Browsed directory.
- Searched for stuff.
- Viewed a commit.
- Viewed a file diff.
- Edited a commit.
- Viewed history.
- Viewed tags.
- Viewed push log.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4245

Differential Revision: https://secure.phabricator.com/D14929

src/applications/diffusion/controller/DiffusionExternalController.php

@@ -109,7 +109,7 @@ final class DiffusionExternalController extends DiffusionController {
             array(
               'href' => $href,
             ),
-              'r'.$repo->getCallsign().$commit->getCommitIdentifier()),
+            $commit->getMonogram()),
           $commit->loadCommitData()->getSummary(),
         );
       }