Generate diff UI lazily for large files

[danvk]

I believe that generating diffs is quite fast, but generating the UI for them may not be. For example, run:

git webdiff 52aa15f^..52aa15f

from https://github.com/danvk/codediff.js and click through to package-lock.json. There's a noticeable pause while it loads the UI. The diff is long and 99+% of it is offscreen. The lower parts could be generated lazily.

This probably isn't even worth trying without React-ifying codediff.js first.