When the diff file index is in docked mode, and the user switches which

page was visible, we would hit an error and the file index would fail to

update as the diff was loading, leaving every file showing a spinner.
The problem is that when the page is scrolled, we redock the items table

elsewhere onto the page. Inside the #update() method we would load all

the new items, but then reset the #$items variable to be tr

(grand)children of the view's element, which was empty because the table

was a child of the banner instead.
I've updated it to explicitly set #$items to be tr children of the

items table. While in here, I realized that the way we're assembling the

items table is quite inefficient, so I fixed this up to assemble all the

child nodes first and then add them all to the DOM at the same time.

Ran js-tests.
Switched around between pages of a very large diff. Made sure that

  the paginators at both the top and bottom of the page worked

  correctly.