Improve diff pagination a little bit.
This doesn't change pagination in a wild way, it just fixes it to not be super

weird. During the diff index changes, the pagination links at the top of the

diffs disappeared, leaving it so the only one was at the bottom. This was a

rude surprise when I slogged through a big review only to discover that there

were two or three more pages! I've restored the pagination links at the top,

which means there's one set of links after the diff index and one at the very

bottom of the page.
I also changed it so switching pages will operate without a reload. This makes

it very smooth to navigate around.

Switched around to a bunch of different revisions and interdiff revisions with

multiple pages. Made sure that switching pages always went to the right place,

and that switching to new revisions always loaded the first page of the

new revision.