Correct rendering of workboard column move stories when a single transaction performs moves on multiple boards

Summary:
See <https://discourse.phabricator-community.org/t/unhandled-exception-rendering-maniphest-task/3234>.

If a single transaction performs column moves on multiple different boards (which is permitted in the API), the rendering logic currently fails. Make it render properly.

Test Plan: {F7011464}

Differential Revision: https://secure.phabricator.com/D20901

src/applications/transactions/storage/PhabricatorApplicationTransaction.php

@@ -1133,6 +1133,8 @@ abstract class PhabricatorApplicationTransaction
         } else {
           $fragments = array();
           foreach ($moves as $move) {
+            $to_column = $move['columnPHID'];
+            $board_phid = $move['boardPHID'];
             $fragments[] = pht(
               '%s (%s)',
               $this->renderHandleLink($board_phid),