Treat MERGE_INTO as a status transaction for mail tags

Summary: Fixes T6838. We use a special transaction type for merging, but don't handle it when figuring out mail tags.

Test Plan: Verified merge mail picks up the `maniphest-status` tag.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T6838

Differential Revision: https://secure.phabricator.com/D11101

src/applications/maniphest/storage/ManiphestTransaction.php

@@ -794,6 +794,7 @@ final class ManiphestTransaction
   public function getMailTags() {
     $tags = array();
     switch ($this->getTransactionType()) {
+      case self::TYPE_MERGED_INTO:
       case self::TYPE_STATUS:
         $tags[] = self::MAILTAG_STATUS;
         break;