Fix two issues with callsign-free repositories

Summary:
Ref T4245. These callsites don't quite do the right thing if a repository has no callsign.

See also <https://github.com/phacility/phabricator/pull/822>.

Test Plan: Made a comment on a commit in a repository with no callsign.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4245

Differential Revision: https://secure.phabricator.com/D15344

src/applications/diffusion/controller/DiffusionExternalController.php

@@ -103,7 +103,7 @@ final class DiffusionExternalController extends DiffusionController {
             array(
               'href' => $href,
             ),
-            $commit->getMonogram()),
+            $commit->getURI()),
           $commit->loadCommitData()->getSummary(),
         );
       }