bug#45029: 27.1.50; Regression: Yanking into externally modified file with delete-selection-mode

[Eli Zaretskii <eliz@gnu.org>]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Date: Fri, 04 Dec 2020 11:18:29 +0100
> Cc: 45029@debbugs.gnu.org
> 
> Lars Ljung <lars@matholka.se> writes:
> 
> > I get an error when I do this, and the content of the buffer is
> > completely replaced by a single "y".
> >
> > 1. Start with "emacs -Q"
> > 2. M-x delete-selection-mode
> > 3. Open some text file (fundamental mode)
> > 4. Modify the file externally
> > 5. While a selection is active, yank some text
> > 6. Answer yes to the question "<file> changed on disk..."
> >
> > The content of the buffer is now completely replaced by a single "y".
> > The message "No catch for tag: exit, nil" is shown in the minibuffer.
> 
> I can confirm that this bug is still present in Emacs 28.
> 
> The backtrace is included below, if that helps anybody.
> 
> Debugger entered--Lisp error: (no-catch exit nil)
>   throw(exit nil)
>   exit-minibuffer()
>   read-char-from-minibuffer-insert-char()
>   funcall-interactively(read-char-from-minibuffer-insert-char)
>   call-interactively(read-char-from-minibuffer-insert-char nil nil)
>   command-execute(read-char-from-minibuffer-insert-char)

I'm guessing this is some unintended consequence of replacing
read-char-choice in userlock.el with read-char-from-minibuffer.

Juri, could you please look into this regression in Emacs 27?  I'd
like to try to fix this for 27.2.

Thanks.