bug#70587: More buffers that ought to be read-only

[Dmitry Gutov]

On 28/04/2024 08:13, Eli Zaretskii wrote:
> > But if we install the above patch, then*vc-diff*  will suddenly not be
> > read-only any more, so we should arguably change
> > `diff-default-read-only` at the same time, which will in turn cause
> > another behavior change when people open a `.diff` file.
> Why should*vc-diff*  be read-only regardless of
> diff-default-read-only?  And why visiting a .diff file is different in
> this regard?

I also think that we should flip diff-default-read-only to true, and 
drop the special casing in vc-diff. :-)