Support short version of arc diff --add-all

Summary: Like `git commit -a`.

Test Plan:
  $ arc diff -a

Reviewers: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4903

src/workflow/ArcanistDiffWorkflow.php

@@ -294,6 +294,7 @@ EOTEXT
         ),
       ),
       'add-all' => array(
+        'short' => 'a',
         'help' =>
           'Automatically add all untracked, unstaged and uncommitted files to '.
           'the commit.',