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

>> 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.

OTOH, there are other packages that do something similar,
e.g. flymake.el:

  (require 'compile) ; for some faces

The only drawback is that 'M-x list-faces-display'
shows these faces with the default colors, but this is
not a problem.  So maybe your latest patch could be pushed.