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.
>
> For me, it reverts it to whatever the permission bits on the file say.
> Which is IMO the correct behaviour for revert-buffer.
> Perhaps you are referring to a short-lived change from
> https://debbugs.gnu.org/35166#24
> which was quickly reverted?

Here is what I tried in Emacs versions 26.3 and 28.0.50:

1. 'C-h C-t' that visits etc/TODO in read-only view-mode
2. 'M-x revert-buffer RET yes RET'

In 26.3 there is no view-mode, but still there is a read-only indicator.
In 28.0.50 no view-mode, no read-only indicator.

3. 'C-x C-q' to enable read-only
4. again 'M-x revert-buffer RET yes RET'

In 26.3 the buffer is still read-only.
In 28.0.50 no read-only indicator.