Improve file-by-file diffs prev/next pagination

Follow-up from the discussion in !35223 (merged) (!35223 (comment 367391248)).

1. This pagination component is different from the one we're using for the commits pagination. Notice the chevron icons. I'm unsure if either of them are using our compact pagination variation.
2. We're missing a tooltip explaining why the Prev or Next buttons are disabled (see commits example). Removed in favor of the page & total label text below the paginator
3. Can we remove the pagination if there's only one file? This would mimic the commits pagination behavior as well.