Clean up the diff loading style.
The diff loading style was a bit broken-looking. The "Revision" and

"New Change" headers jumped on load, and the spinner had no padding

around it.
It's impossible to properly align the revision headers without knowing

some of the visible line numbers and content and placing it in the

table, and any best-guesses are just guesses. The next best thing is to

just not show revision information until the file has loaded, which is

what we do now. It's a bit cleaner-looking, with just a filename and a

spinner (which now has some padding around it as well).

Temporarily delayed loading to better see the style on a local setup.

It looked a lot cleaner when loading and transitioning to a loaded state.