bug#81583: 32.0.50; Optional preview of the replacement text while typing it

> I made it closer to Karthik's screenshot, also added the diff faces as
> base for the feature.  Much better.

Thanks, indeed it's much better.

> The diff faces are not autoloaded, so a top-level `require` would load
> them even when they aren't needed.  That's why the `require` is in
> `replace-preview-setup`, in the branch that only runs when the preview
> is enabled. Is this acceptable?

Requiring diff-mode just for faces doesn't look right.

> The alternative would be to copy the color values from `diff-mode.el`
> into the two faces.  That would avoid the load, but it would duplicate
> the colors and ignore any customizations the user or theme has made to
> the diff faces.

Duplication of face definitions doesn't look right either.

What we can do is to add new generic faces in faces.el
by copying face definitions from diff-mode, then inherit
from them in packages.  But unfortunately this will lose
any customizations by users.

I don't remember why we don't use ###autoload for defface.
But adding ###autoload to these two faces in diff-mode.el
could solve the problem.

> An extra: Zero-length matches are better than before (at least for the
> 'both case), since an empty match at least shows the arrow, so there is
> something at the spot.  I still think it deserves a change of its own
> when we finish this part of the feature.

Yes, for the value 'both' a placeholder for an empty match is not much needed.
But still it would be better to have it at least for the 't' case.