Fix a transaction editor "continue;" inside "switch()" for PHP 7.3

Summary: See <https://discourse.phabricator-community.org/t/new-git-commit-processing-fails-on-php-7-3/>. This "continue" should be a "break".

Test Plan:
{F6045490}

  - Tried to assign a task to myself while I was already the owner, got an appropriate error.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D19844

src/applications/maniphest/editor/ManiphestTransactionEditor.php

@@ -649,7 +649,7 @@ final class ManiphestTransactionEditor
         $old_value = $object->getOwnerPHID();
         $new_value = $xaction->getNewValue();
         if ($old_value === $new_value) {
-          continue;
+          break;
         }
 
         // When a task is reassigned, move the old owner to the subscriber