bug#64071: 28.2; smerge-diff creates *vc-diff* without setting it up

[Eli Zaretskii]

> From: Spencer Baugh <sbaugh@janestreet.com>
> Cc: Stefan Monnier <monnier@iro.umontreal.ca>,  64071@debbugs.gnu.org
> Date: Tue, 27 Jun 2023 16:17:45 -0400
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > That command enters diff-mode, and diff-mode doesn't force read-only
> > status on the current buffer.  Why should Smerge force that?
> 
> Buffer named "*vc-diff*" are universally read-only, except if they are
> created through this path.

But users can legitimately make a *vc-diff* buffer modifiable, no?
And your patch silently makes it read-only again, behind the user's
back.  That's hardly nice, is it?

> Stefan likes my simple patch, so what's wrong with it?

See above: we shouldn't change the read-only attribute that was set by
the user.