Here's a revised patch. I've been running with this for a few days, and
it feels delightful. Changes from the last time:

- Removed the unnecessary (call-interactively ...) pointed out by Eli

- Instead of using (easy-mmode-define-navigation) to create
  diff-{hunk,file}-{next,prev} and then overriding that definition, I
  now have (easy-mmode-define-navigation) create
  diff--internal-{hunk,file}-{next,prev}, and then my own
  (diff-{hunk,file}-{next,prev}) call those functions

- I now handle file navigation in addition to hunk navigation