Here's yet another revised patch. The (call-interactively) at the end of
(diff-apply-hunk) was important, it turns out. This would force it to
use the new logic to move to the next hunk, instead of the legacy logic.
I purposely left the behavior of (diff-next-hunk) unchanged from before
when running non-interactively, and here I explicitly want the new
behavior.

This patch puts the (call-interactively) back in, and explains the
rationale.