Reduce the frequency of DOM scans to rebuild inlines when scrolling revisions

Summary:
Ref T13513. See PHI1734, which raises a concern about the performance of large revisions near the 100-change threshold.

Currently, `getInlines()` is called whenever the scroll position transitions between two changesets, and it performs a relatively complicated DOM scan to lift inlines out of the document.

This shows up as taking a small but nontrivial amount of time in Firefox profiles and should be safely memoizable.

Test Plan:
  - Under Firefox profiling, scrolled through a large revision.
  - Before change: `getInlines()` appeared as the highest-cost thing we're explicitly doing on profiles.
  - After change: `getInlines()` was no longer meaningfully represented on profiles.
  - Created inlines, edited inlines, etc. Didn't identify any broken behavior.

Maniphest Tasks: T13513

Differential Revision: https://secure.phabricator.com/D21261

resources/celerity/map.php

@@ -13,7 +13,7 @@ return array(
     'core.pkg.js' => '845355f4',
     'dark-console.pkg.js' => '187792c2',
     'differential.pkg.css' => '319dca29',
-    'differential.pkg.js' => '695827fc',
+    'differential.pkg.js' => 'bb2a17fc',
     'diffusion.pkg.css' => '42c75c37',
     'diffusion.pkg.js' => 'a98c0bf7',
     'maniphest.pkg.css' => '35995d6d',
@@ -379,7 +379,7 @@ return array(
     'rsrc/js/application/dashboard/behavior-dashboard-move-panels.js' => 'a2ab19be',
     'rsrc/js/application/dashboard/behavior-dashboard-query-panel-select.js' => '1e413dc9',
     'rsrc/js/application/dashboard/behavior-dashboard-tab-panel.js' => '0116d3e8',
-    'rsrc/js/application/diff/DiffChangeset.js' => 'bfdae878',
+    'rsrc/js/application/diff/DiffChangeset.js' => 'e02670b5',
     'rsrc/js/application/diff/DiffChangesetList.js' => 'b1b8500b',
     'rsrc/js/application/diff/DiffInline.js' => 'b00168c1',
     'rsrc/js/application/diff/DiffPathView.js' => '8207abf9',
@@ -774,7 +774,7 @@ return array(
     'phabricator-darklog' => '3b869402',
     'phabricator-darkmessage' => '26cd4b73',
     'phabricator-dashboard-css' => '5a205b9d',
-    'phabricator-diff-changeset' => 'bfdae878',
+    'phabricator-diff-changeset' => 'e02670b5',
     'phabricator-diff-changeset-list' => 'b1b8500b',
     'phabricator-diff-inline' => 'b00168c1',
     'phabricator-diff-path-view' => '8207abf9',
@@ -2019,19 +2019,6 @@ return array(
     'bcec20f0' => array(
       'phui-theme-css',
     ),
-    'bfdae878' => array(
-      'javelin-dom',
-      'javelin-util',
-      'javelin-stratcom',
-      'javelin-install',
-      'javelin-workflow',
-      'javelin-router',
-      'javelin-behavior-device',
-      'javelin-vector',
-      'phabricator-diff-inline',
-      'phabricator-diff-path-view',
-      'phuix-button-view',
-    ),
     'c03f2fb4' => array(
       'javelin-install',
     ),
@@ -2124,6 +2111,19 @@ return array(
     'df3afa61' => array(
       'phui-inline-comment-view-css',
     ),
+    'e02670b5' => array(
+      'javelin-dom',
+      'javelin-util',
+      'javelin-stratcom',
+      'javelin-install',
+      'javelin-workflow',
+      'javelin-router',
+      'javelin-behavior-device',
+      'javelin-vector',
+      'phabricator-diff-inline',
+      'phabricator-diff-path-view',
+      'phuix-button-view',
+    ),
     'e150bd50' => array(
       'javelin-behavior',
       'javelin-stratcom',

webroot/rsrc/js/application/diff/DiffChangeset.js

@@ -817,11 +817,18 @@ JX.install('DiffChangeset', {
     },
 
     getInlines: function() {
+      if (this._inlines === null) {
         this._rebuildAllInlines();
+      }
+
       return this._inlines;
     },
 
     _rebuildAllInlines: function() {
+      if (this._inlines === null) {
+        this._inlines = [];
+      }
+
       var rows = JX.DOM.scry(this._node, 'tr');
       var ii;
       for (ii = 0; ii < rows.length; ii++) {