Reorder "Merge" transaction to make "Close as Duplicate" produce a "[Merged]" email subject

Summary:
Fixes T11782. When you "Close as Duplicate", generate a "[Merged]" email by making the merge the first transaction.

(There are other, more-deterministic ways to do this with action strength, but this is much simpler and I believe it suffices.)

Test Plan: Used "Close as Duplicate", got a "[Merged]" email out of it.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T11782

Differential Revision: https://secure.phabricator.com/D19972

src/applications/maniphest/relationship/ManiphestTaskRelationship.php

@@ -31,13 +31,12 @@ abstract class ManiphestTaskRelationship
     $subscriber_phids = $this->loadMergeSubscriberPHIDs($tasks);
 
     $xactions[] = id(new ManiphestTransaction())
-      ->setTransactionType(PhabricatorTransactions::TYPE_SUBSCRIBERS)
-      ->setNewValue(array('+' => $subscriber_phids));
+      ->setTransactionType(ManiphestTaskMergedFromTransaction::TRANSACTIONTYPE)
+      ->setNewValue(mpull($tasks, 'getPHID'));
 
     $xactions[] = id(new ManiphestTransaction())
-        ->setTransactionType(
-          ManiphestTaskMergedFromTransaction::TRANSACTIONTYPE)
-        ->setNewValue(mpull($tasks, 'getPHID'));
+      ->setTransactionType(PhabricatorTransactions::TYPE_SUBSCRIBERS)
+      ->setNewValue(array('+' => $subscriber_phids));
 
     return $xactions;
   }