bug#50431: revert-buffer doesn't preserve read-onlyness

[Juri Linkov]

>> I don't know at what moment this regression occurred,
>> but now M-x revert-buffer in a read-only file buffer
>> doesn't keep it read-only.
>
> I've now fixed this in Emacs 29 -- `C-x C-q' records what the user
> actually did, so we can restore that state.
>
> If the user hasn't used that command, then reverting works as before --
> it uses the permission bits on the file and updates the readedness
> state.
>
> I considered only heeding the user-set readedness state if it was
> different from the default one -- that is, if the user used the `C-x
> C-q' command twice, then readedness state would not be preserved by
> reverting, but I think that's too subtle.

I confirm that now the read-only status is restored,
but reverting loses view-mode:

  C-h C-t  -- note "View" on the mode-line;
  C-x x g  -- "View" is lost.