Fix visibility for DifferentialManiphestTasksField::readValueFromRevision

Summary: Ref T6822.

Test Plan: `grep` for `->readValueFromRevision(`.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6822

Differential Revision: https://secure.phabricator.com/D11364

src/applications/differential/customfield/DifferentialManiphestTasksField.php

@@ -35,7 +35,7 @@ final class DifferentialManiphestTasksField
     return $this->getFieldName();
   }
 
-  public function readValueFromRevision(DifferentialRevision $revision) {
+  protected function readValueFromRevision(DifferentialRevision $revision) {
     if (!$revision->getPHID()) {
       return array();
     }