Allow --verbatim and -m in arc diff

Summary:
There's no reason these should be exclusive: -m is used only on update for the
update message, and --verbatim doesn't affect the udpate message. It's also
useful to allow both of these, if say I want to update my test plan by editing
my git commit message and also provide a message for the update from the
command line.

Test Plan:
ran arc diff with --verbatim and -m and saw my message from -m was used, as
well as my updates in the commit message went through.

Reviewers: jungejason, epriestley, vrana

Reviewed By: epriestley

CC: aran, Koolvin

Differential Revision: https://secure.phabricator.com/D2426

src/workflow/diff/ArcanistDiffWorkflow.php

@@ -315,7 +315,6 @@ EOTEXT
           'preview'             => true,
           'raw'                 => true,
           'raw-command'         => true,
-          'message'             => true,
           'message-file'        => true,
         ),
       ),