bug#32991: 27.0.50; diff-auto-refine-mode a no-op

[Stefan Monnier]

>> This change seems unrelated.  I'd rather we stick to the refinement itself.
> Without this change, other functions in diff-mode (such as
> diff--font-lock-syntax) calling diff-hunk-next accidentally trigger
> hunk refinement if 'diff-refine' is 'navigation'.

Ah, right, makes sense.  Could we fix this more directly by using
`called-interactively` instead?

> Incidentally, I left out the auto-recentering and buffer
> restriction-changing parts of the old diff-hunk-next and
> diff-hunk-prev, since these behaviors do not match other navigation
> commands.

Indeed, I see several changes in there, which is why I'd rather we
separate them into another patch.

[ FWIW I never used the restriction (I coded it up only to mach the
  featureset of some earlier diff-mode I'd found somewhere), but I would
  miss the recentering.  More to the point, rather than removing the
  recentering, I'd like to improve it (so it tries harder to make the
  whole hunk visible when possible).  ]

Other than that, LGTM.


        Stefan