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

[Lars Ingebrigtsen]

Juri Linkov <juri@linkov.net> writes:

> 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.