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

[Stefan Monnier <monnier@iro.umontreal.ca>]

> So diff-font-lock-refine could have 3 possible values, t and nil as it
> has now, and 'auto for doing the refinement as you navigate to hunks
> with "n" or "p".

I don't see what the navigation-triggered refinement has of
"auto"matism, compared to font-lock, so I wouldn't use `auto` here.
I'd rather go with something like `nil`, `font-lock`, or `navigation`
(and default to `font-lock`).

> While we're on this point, what is the use case for offering automatic
> refining during navigation if we can now offer "just-in-time"
> highlighting via font-lock?

Good question.  Maybe it's just not worth it and we should simply get
rid of the old navigation-triggered refinement.  This said, the new
font-lock thingy can be problematic if the diff takes too much time
since it happens within jit-lock with inhibit-quit set to a non-nil
value, so it completely freezes your Emacs session, whereas if it
happens during `n` you can stop it with C-g.

Hopefully, we can fix this problem by calling `diff` asynchronously so
it can't block Emacs.

> It could be even better if C-c C-b could interactively toggle the
> refining of the hunk at point (for those times when the refining turns
> out to be an eye-sore).

Sounds good (but note that diff-refine-hunk can also be useful to
*refresh* the fine highlighting, e.g. after manually editing a hunk).

> Another advantage of the new font-lock based approach is that with a
> little change we might be able to customize how much highlighting is
> done in a diff-mode buffer via "font-lock-maximum-decoration".

font-lock-maximum-decoration is fundamentally flawed in that it is
unidimensional (you can only have more or less) whereas often some users
may want more of one kind of info and less of another.

E.g. how would you order the "decoration levels" between:

    basic
    basic + refine
    basic + prettify
    basic + prettify + refine

The first and last are easy, but there's no natural ordering between the
middle two.


        Stefan