Fix slowvote exception when viewing description diff

Summary: Fixes T6937. We weren't passing required parameters.

Test Plan: Followed repro steps in task.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T6937

Differential Revision: https://secure.phabricator.com/D11346

src/applications/slowvote/storage/PhabricatorSlowvoteTransaction.php

@@ -146,7 +146,10 @@ final class PhabricatorSlowvoteTransaction
   }
 
   public function renderChangeDetails(PhabricatorUser $viewer) {
-    return $this->renderTextCorpusChangeDetails($viewer);
+    return $this->renderTextCorpusChangeDetails(
+      $viewer,
+      $this->getOldValue(),
+      $this->getNewValue());
   }