Clicking a file name should scroll to its diff

[danvk]

When you have a really long, expanded file list (tall enough that it covers the entire screen), it's a bit surprising that clicking a file name results in no visible changes on the page. The diff does change, but it's off-screen. It would make more sense if clicking the file name scrolled you down to the diff.

This is tricky to get right—scrolling down to the diff might be surprising behavior if there are just a few files and the diff itself is on-screen.