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

[Juri Linkov]

>> >> Maybe minor modes should be restored after reverting?
>> >
>> > Only the minor modes that are automatically turned on when visiting
>> > the file.  Reverting should produce the same effect as killing the
>> > buffer and then visiting the file anew, nothing more, nothing less.
>>
>> In the above example, `C-h C-t' `C-x x g' doesn't preserve the same state
>> as visiting the file anew with `C-h C-t'.
>>
>> > For that reason, I'm not sure the change that preserves the read-only
>> > status across reverts is correct.  At the very least, it should be an
>> > opt-in feature, IMO.
>>
>> It was a regression when reverting stopped to preserve the read-only status
>> that worked in older versions.
>
> IMO, that was a mistake.  It violates the notion of "reverting", at
> least in my book.

Maybe a new user option could define how to revert, with possible values:

1. keep the initial read-only status and minor modes;
2. keep the current read-only status and minor modes.