Diffusion - fix browse file bug

Summary: we were calling a member method on a diffusion hash. not sure why.  Fixes T4402

Test Plan: clicked about, no fatals and seemed to move sensical backwards in time

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T4402

Differential Revision: https://secure.phabricator.com/D8194

src/applications/diffusion/controller/DiffusionBrowseFileController.php

@@ -891,7 +891,7 @@ final class DiffusionBrowseFileController extends DiffusionBrowseController {
       $follow = 'created';
     } else if ($parent) {
       // If we found a parent, jump to it. This is the normal case.
-      $target_commit = $parent->getCommitIdentifier();
+      $target_commit = $parent;
     } else {
       // If there's no parent, this was probably created in the initial commit?
       // And the "was_created" check will fail because we can't identify the