Fix Back to HEAD link

Summary: I missed an anchor tag here, adds it back

Test Plan: View blame, click a previous version of the file, click Back to HEAD link.

Reviewers: epriestley

Reviewed By: epriestley

Spies: Korvin

Differential Revision: https://secure.phabricator.com/D18451

src/applications/diffusion/controller/DiffusionBrowseController.php

@@ -1601,6 +1601,7 @@ final class DiffusionBrowseController extends DiffusionController {
     $head = null;
     if ($behind_head) {
       $head = id(new PHUIButtonView())
+        ->setTag('a')
         ->setText(pht('Back to HEAD'))
         ->setHref($head_uri)
         ->setIcon('fa-home')