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

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

> 1. emacs -Q
> 2. Create a buffer containing:
> <<<<<<< left
> foo
> =======
> bar
>>>>>>>> right
> 3. M-x smerge-mode
> 4. M-x smerge-diff-upper-lower
> 5. The resulting *vc-diff* buffer is in diff-mode but it's not
> read-only, which is unusual for a *vc-diff* buffer.  Probably there is
> also other setup missing.

Good catch, indeed.

> Relatedly, if a *vc-diff* buffer already exists, smerge-mode will use it
> without updating the mode-line, so it will say, for example, "Diff from
> *vc-change-log*" despite the diff being from smerge conflict resolution.

Hmm... that's also a bug, indeed.  I'm tempted to say that maybe the bug
is in the code which changed the `mode-line-format` (it should arguably
either not change it, or arrange to have it reset when it doesn't match
the buffer's contents any more).

> This bug is also present in Emacs 29 and trunk.
>
> The below diff resolves the read-only part.  But maybe we want to be
> creating the *vc-diff* by calling into vc, which would also fix the
> mode-line part?
>
> diff --git a/lisp/vc/smerge-mode.el b/lisp/vc/smerge-mode.el
> index c39a9cc2f22..65d93c17a64 100644
> --- a/lisp/vc/smerge-mode.el
> +++ b/lisp/vc/smerge-mode.el
> @@ -1259,6 +1259,7 @@ smerge-diff
>  	      (if (eq status 0) (insert "No differences found.\n"))))
>  	  (goto-char (point-min))
>  	  (diff-mode)
> +          (read-only-mode)
>  	  (display-buffer (current-buffer) t))
>        (delete-file file1)
>        (delete-file file2))))

LGTM.


        Stefan