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

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

> Stefan, this seems to be due to your changes in b2e4481906ec: you
> added '(setq buffer-read-only t)' which was not there before (in fact,
> the code before those changes did the opposite:
>
> --- a/lisp/vc/diff.el
> +++ b/lisp/vc/diff.el
> @@ -131,37 +147,32 @@
> -        (thisdir default-directory)
> -        proc)
> -    (save-excursion
> -      (display-buffer buf)
> -      (set-buffer buf)
> -      (setq buffer-read-only nil)
> +    (with-current-buffer buf
> +      (setq buffer-read-only t)
>
> So I suggest to remove the setting of buffer-read-only there'
> diff-mode will do TRT according to diff-default-read-only.

There is no right or wrong answer here.  The current design reflects the
behavior I think makes more sense, but that's largely just my
personal preference.  The above patch was no mistake.

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.


        Stefan