Fix an issue where scrolling down, then up, then down fails to show changeset header in Differential

Summary: Ref T13151. See PHI616. There's a bug where the current banner changeset isn't cleared correctly when we hide the banner.

Test Plan:
  - View revision with several changesets.
  - Scroll down slowly through first changeset until banner appears.
  - Scroll up until banner disappears.
  - Scroll back down.
  - Before: banner fails to reappear (code still thinks it's visible and we don't want to update it).
  - After: banner reappears correctly.

Reviewers: amckinley, jmeador

Reviewed By: jmeador

Maniphest Tasks: T13151

Differential Revision: https://secure.phabricator.com/D19474

webroot/rsrc/js/application/diff/DiffChangesetList.js

@@ -1381,6 +1381,7 @@ JX.install('DiffChangesetList', {
       var changeset = this._getVisibleChangeset();
 
       if (!changeset) {
+        this._bannerChangeset = null;
         JX.DOM.remove(node);
         return;
       }