Make the filetree view width sticky across show/hide and reload

Summary:
Ref T13090. The default width changed recently to become much wider, but the behavior on this control isn't great. Instead:

  - Pick a default width somewhere between the two.
  - Make the width sticky across show/hide (pressing "f" twice remembers your width instead of resetting it).
  - Make the width sticky across reloads (dragging the bar, then reloading the page keeps the bar in the same place).

Test Plan:
  - Without settings, loaded page: got medium-width bar.
  - Dragged bar wide/narrow, toggled on/off with "f", got persistent width.
  - Dragged bar wide/narrow, reloaded page, got persistent width.
  - Dragged bar wide/narrow, toggled it off, reloaded page, toggled it on, got persistent width.

Maniphest Tasks: T13090

Differential Revision: https://secure.phabricator.com/D19129

src/applications/diffusion/controller/DiffusionCommitController.php

@@ -415,17 +415,21 @@ final class DiffusionCommitController extends DiffusionController {
       PhabricatorShowFiletreeSetting::SETTINGKEY,
       PhabricatorShowFiletreeSetting::VALUE_ENABLE_FILETREE);
 
-    $pref_collapse = PhabricatorFiletreeVisibleSetting::SETTINGKEY;
-    $collapsed = $viewer->getUserSetting($pref_collapse);
-
     $nav = null;
     if ($show_changesets && $filetree_on) {
+      $pref_collapse = PhabricatorFiletreeVisibleSetting::SETTINGKEY;
+      $collapsed = $viewer->getUserSetting($pref_collapse);
+
+      $pref_width = PhabricatorFiletreeWidthSetting::SETTINGKEY;
+      $width = $viewer->getUserSetting($pref_width);
+
       $nav = id(new DifferentialChangesetFileTreeSideNavBuilder())
         ->setTitle($commit->getDisplayName())
         ->setBaseURI(new PhutilURI($commit->getURI()))
         ->build($changesets)
         ->setCrumbs($crumbs)
-        ->setCollapsed((bool)$collapsed);
+        ->setCollapsed((bool)$collapsed)
+        ->setWidth((int)$width);
     }
 
     $view = id(new PHUITwoColumnView())