bug#65854: Multi-file replacement diff

[Juri Linkov]

>> >> +                                                    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.