bug#65854: Multi-file replacement diff

[Eli Zaretskii]

> From: Juri Linkov <juri@linkov.net>
> Cc: me@eshelyaron.com,  65854@debbugs.gnu.org
> Date: Sat, 23 Sep 2023 20:36:02 +0300
> 
> >> >> +                                                    However, when the 
> >> >> file
> >> >> +is not visited in a buffer, or the buffer is not modified, still read
> >> >> +contents from the file."
> >> >
> >> > Seems to describe an implementation detail, and I don't think it
> >> > should be there.  E.g., what if the file visited by the buffer no
> >> > longer exists?
> >>
> >> If the file visited by the buffer no longer exists, then
> >> the standard error is signaled.
> >
> > Which means in that case it is better to use the buffer text, no?
> 
> Since replacement diffs are not supported in non-file buffers,
> better to signal an error for heads up.

But it _is_ a file-visiting buffer.  It's just that its file was
deleted meanwhile.