Fix default order for Differential queries

Summary:
Fixes T3781. The UI defaults to "Created" but the query defaults to "Modified". Make the two consistent.

In particular, an issue this fixes is that previously a `/differential/?authors=duck` page would show "Order: Created" but actually order by "Modified".

Test Plan: Visited `/differential/?authors=duck` and verified the revisions were ordered by creation date.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3781

Differential Revision: https://secure.phabricator.com/D6843

src/applications/differential/query/DifferentialRevisionSearchEngine.php

@@ -82,6 +82,8 @@ final class DifferentialRevisionSearchEngine
     $order = $saved->getParameter('order');
     if (idx($this->getOrderOptions(), $order)) {
       $query->setOrder($order);
+    } else {
+      $query->setOrder(DifferentialRevisionQuery::ORDER_CREATED);
     }
 
     return $query;