Make 'arc diff --auto' only identify open revisions

Summary: Otherwise, "arc diff --auto" on a branch with the same name as some
other branch you previously used tries to update that revision.

Test Plan: Ran "arc diff --auto" on a "listdocs" branch; arc didn't try to
update D1639.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T614

Differential Revision: https://secure.phabricator.com/D1662

src/workflow/diff/ArcanistDiffWorkflow.php

@@ -1127,6 +1127,7 @@ EOTEXT
         $this->getConduit(),
         array(
           'authors' => array($this->getUserPHID()),
+          'status'  => 'status-open',
         ));
       if (!$revisions) {
         $is_create = true;