Move commits to the "COMMIT" mail prefix

Summary:
Fixes T9427. Currently, replies to audits/commits go to "Cxxx", but so do replies to countdowns.

There is non real non-disruptive approach available here and this seems least-bad.

Test Plan:
- Made a comment on a commit.
- Fished the reply-to address out of `bin/mail list-oubound` + `bin/mail show-outbound` (it was now "COMMIT...").
- Sent mail to that address.
- Grabbed the raw message and wrote it to `mail.txt`.
- Ran `cat mail.txt | ./scripts/mail/mail_handler.php --process-duplicates`.
- Used `bin/mail list-inbound` + `bin/mail show-inbound` to verify receipt.
- Saw comment appear on audit.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9427

Differential Revision: https://secure.phabricator.com/D14163

src/applications/audit/mail/PhabricatorAuditMailReceiver.php

···
   }
 
   protected function getObjectPattern() {
-    return 'C[1-9]\d*';
+    return 'COMMIT[1-9]\d*';
   }
 
   protected function loadObject($pattern, PhabricatorUser $viewer) {
-    $id = (int)trim($pattern, 'C');
+    $id = (int)preg_replace('/^COMMIT/', '', $pattern);
 
     return id(new DiffusionCommitQuery())
       ->setViewer($viewer)

src/applications/audit/mail/PhabricatorAuditReplyHandler.php

···
   }
 
   public function getObjectPrefix() {
-    // TODO: This conflicts with Countdown and will probably need to be
-    // changed eventually.
-    return 'C';
+    return 'COMMIT';
   }
 
 }