Fix performance issue with large changesets in Differential

Summary:
Fixes T3151. Javelin treats a behavior without parameters as a global behavior and invokes it only once no matter how many times it is initialized (this is necessarily correct for any reasonable behavior, as the inputs do not vary). A recent patch changed `differential-dropdown-menus` from a zero-argument global behavior to an implicitly nonzero-argument behavior by adding `pht`.

Currently, we initialize the behavior next to dropdown menu creation, so this resulted in `O(N^2)` initializations of the menus. For large diffs, this locks browsers. Instead, initialize outside of the dropdown loop so we ginitialize each menu just once.

Test Plan: Viewed a 2,000 file diff without browser lock.

Reviewers: wez, vrana, btrahan

Reviewed By: wez

CC: aran

Maniphest Tasks: T3151

Differential Revision: https://secure.phabricator.com/D5885

src/applications/differential/view/DifferentialChangesetListView.php

@@ -108,6 +108,9 @@ final class DifferentialChangesetListView extends AphrontView {
     $changesets = $this->changesets;
 
     Javelin::initBehavior('differential-toggle-files', array());
+    Javelin::initBehavior(
+      'differential-dropdown-menus',
+      array());
 
     $output = array();
     $mapping = array();
@@ -304,10 +307,6 @@ final class DifferentialChangesetListView extends AphrontView {
 
     $meta['containerID'] = $detail->getID();
 
-    Javelin::initBehavior(
-      'differential-dropdown-menus',
-      array());
-
     return javelin_tag(
       'a',
       array(