Work around workflow blocking error with duplicate "master" refs in "Land Revision"

Summary:
Ref T11823. See PHI68. T11823 has a full description of this issue and a plan to fix it, but the full plan is relatively complicated.

Until that can happen, provide a workaround for the biggest immediate issue, where multiple copies of a ref cursor can cause `executeOne()` to throw, since it expects a single result. In practice, these copies are always identical so we can just pick the first one.

This will get cleaned up once T11823 is fixed properly.

Test Plan:
Forced the table into a duplicate/ambiguous state, reproduced a similar-looking error:

{F5180999}

Applied the patch, got the "Land" to work as expected:

{F5181000}

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T11823

Differential Revision: https://secure.phabricator.com/D18599

src/applications/differential/controller/DifferentialRevisionOperationController.php

@@ -137,9 +137,19 @@ final class DifferentialRevisionOperationController
       return null;
     }
 
-    return $this->newRefQuery($repository)
+    // NOTE: See PHI68. This is a workaround to make "Land Revision" work
+    // until T11823 is fixed properly. If we find multiple refs with the same
+    // name (normally, duplicate "master" refs), just pick the first one.
+
+    $refs = $this->newRefQuery($repository)
       ->withRefNames(array($default_name))
-      ->executeOne();
+      ->execute();
+
+    if ($refs) {
+      return head($refs);
+    }
+
+    return null;
   }
 
   private function getDefaultRefName(