bug#43007: 27.1; M-j doesn't behave properly in comments

[Eli Zaretskii]

> From: Alessio Vanni <vannilla@firemail.cc>
> Date: Sun, 23 Aug 2020 23:53:37 +0200
> 
> I normally use M-j to continue a long comment over multiple lines.
> Recently, this feature broke in a rather weird way: new lines are opened
> as a comment, but the new line is placed above the line with the point,
> rather than below.  In some other cases, the line appears in the correct
> place, but opening a new line results in a '*' character instead of a
> comment (the character is used on modes where '*' is part of a comment,
> like `js-mode' or `cc-mode'.)
> 
> This happens when running "emacs -Q" too.
> 
> Example:
> 
> // some comment.
> 
> Becomes:
> 
> //
> // some comment.
> 
> Alternatively:
> 
> // some comment.
> //
> //
>   *
>   *
>   *
>   *
> 
> (the '*' just keeps happening for every M-j.)

Alan, please look into this.  Lars says he can reproduce, but I
cannot, at least not in "emacs -Q" with Emacs 27.1.  Can you?