Only associate branch with task if maniphest is enabled

Summary:
Previously, maniphest tasks would get upated by diffs on branches
with tasky names, even if maniphest was disabled.

Test Plan:
Tested createing a diff in sandbox with maniphest disabled, on a
git branch named using the format "t###".  Without this change,
if there happened to be a task in the maniphest DB which matched,
it was updated an email was sent to users.

Reviewers: epriestley

Reviewed By: epriestley

CC: wez, slawekbiel, whhone, Korvin, aran

Differential Revision: https://secure.phabricator.com/D6881

src/applications/differential/field/specification/DifferentialBranchFieldSpecification.php

@@ -47,6 +47,11 @@ final class DifferentialBranchFieldSpecification
   }
 
   public function didWriteRevision(DifferentialRevisionEditor $editor) {
+    $maniphest = 'PhabricatorApplicationManiphest';
+    if (!PhabricatorApplication::isClassInstalled($maniphest)) {
+      return;
+    }
+
     $branch = $this->getDiff()->getBranch();
     $match = null;
     if (preg_match('/^T(\d+)/i', $branch, $match)) { // No $ to allow T123_demo.