feature proposal: n/N keyboard commands should take location into account

[llimllib]

If you scroll down a large PR to the third file (for example), then hit n, diffnav currently switches into single-file mode and brings you back to the first file in the diff.

Instead, n should start on the file you're currently looking at (which is a bit tricky, there may be multiple files on the screen, and a decision needs to be made there).

In the current usage, the loss of current position is pretty jarring