When I made the diff revision selector operate without reloading the page, I

neglected the "Download Diff" link at the top of the page. This link should

only be shown when the page is displaying a full revision, and not when there's

an interdiff.
I've changed it to use an inline style to hide the link on the first page load

if appropriate, and JS to show/hide it when the displayed revision changes.

Played around with the revision selector and watched the link show and hide

itself. Verified that if I initially loaded an interdiff, that the link was

present in the DOM to show later.