Clean up initialization of Differential Show More Behavior in Maniphest

Summary:
add a static variable to the method and use it so we don't init more
than once!

Test Plan:
add a "phlog" and noted only init'd one time.   verified "show more"
links worked correctly.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Maniphest Tasks: T666

Differential Revision: https://secure.phabricator.com/D1553

src/aphront/default/configuration/AphrontDefaultApplicationConfiguration.php

@@ -167,6 +167,8 @@ class AphrontDefaultApplicationConfiguration
           'edit/(?P<id>\d+)/$' => 'ManiphestTaskEditController',
           'descriptionchange/(?P<id>\d+)/$' =>
             'ManiphestTaskDescriptionChangeController',
+          'descriptiondiff/$' =>
+            'ManiphestTaskDescriptionDiffController',
           'descriptionpreview/$' =>
             'ManiphestTaskDescriptionPreviewController',
         ),