Add a preference to completely disable the file tree

Summary:
See D4812.

  - This preference disables the file tree completely.
  - It defaults off, so users who want it will have to go turn it on.
  - Maybe slightly cleaner would be doing this if the tree was hidden and then ajaxing it in if you press "F", but that's complicated and I don't want to bother.
  - Generally, I think this element is useful to something like 5% of users and not useful to 95%.

Test Plan: Enabled and disabled file tree. Looked at commits and revisions; verified they reflected the setting correctly.

Reviewers: vrana

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D4813

src/applications/diffusion/controller/DiffusionCommitController.php

@@ -324,13 +324,20 @@ final class DiffusionCommitController extends DiffusionController {
       'commit' => true,
     ));
 
-    if ($changesets) {
+    $prefs = $user->loadPreferences();
+    $pref_filetree = PhabricatorUserPreferences::PREFERENCE_DIFF_FILETREE;
+    $pref_collapse = PhabricatorUserPreferences::PREFERENCE_NAV_COLLAPSED;
+    $show_filetree = $prefs->getPreference($pref_filetree);
+    $collapsed = $prefs->getPreference($pref_collapse);
+
+    if ($changesets && $show_filetree) {
       $nav = id(new DifferentialChangesetFileTreeSideNavBuilder())
         ->setAnchorName('top')
         ->setTitle($short_name)
         ->setBaseURI(new PhutilURI('/'.$commit_id))
         ->build($changesets)
         ->setCrumbs($crumbs)
+        ->setCollapsed((bool)$collapsed)
         ->appendChild($content);
       $content = $nav;
     } else {