Add square brackets around new revision review requests

Summary:
These didn't get covered when we added square brackets to the rest of the emails
(so gmail can thread them properly).

Test Plan:
Created a local diff, got an email with brackets.

Reviewed By: tuomaspelkonen
Reviewers: rm, tuomaspelkonen, jungejason, aran
CC: aran, epriestley, tuomaspelkonen
Differential Revision: 256

src/applications/differential/mail/newdiff/DifferentialNewDiffMail.php

@@ -29,7 +29,10 @@ class DifferentialNewDiffMail extends DifferentialReviewRequestMail {
       $verb = 'Updated';
     }
 
-    return "{$verb} ({$lines}): ".$revision->getTitle();
+    $revision_id = $revision->getID();
+    $revision_title = $revision->getTitle();
+
+    return "[{$verb}, {$lines}] D{$revision_id}: {$revision_title}";
   }
 
   protected function buildSubject() {