Closing as a duplicate should use "closed" iconography

Summary: Uses the check icon for closed, which is the primary action taken.

Test Plan:
Close as a duplicate, seen new icon.

{F236048}

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D10882

src/applications/maniphest/storage/ManiphestTransaction.php

@@ -352,6 +352,7 @@ final class ManiphestTransaction
         return 'fa-columns';
 
       case self::TYPE_MERGED_INTO:
+        return 'fa-check';
       case self::TYPE_MERGED_FROM:
         return 'fa-compress';