bug#65854: Multi-file replacement diff

[Juri Linkov]

> Yes, but what is the use case for generating the diff based on the
> contents on disk when the file is modified in Emacs?  Basically, my
> suggestion is to check in `multi-file-replace-regexp-as-diff` if any of
> the matching files are visited by some buffer, and if so simply pass the
> buffer instead of the file name for that file to
> `multi-file-replace-as-diff`.  That way you always get an up-to-date
> diff, and you don't need to manually check that you don't have any of
> the matching files open by any chance.  Does that make sense?

Thanks for the idea, this makes sense and will help to
reduce the number of commands from 2 to 1 by merging
multi-buffer-replace-regexp-as-diff with
multi-file-replace-regexp-as-diff.