bug#22520: 25.0.50; unhighlight-regexp shows the scary guts of char-folding to the user

[Dima Kogan <dima@secretsauce.net>]

Hi. I'm running a very recent (1-week-old) build of emacs from

  https://github.com/emacs-mirror/emacs/commit/da976cf

I've been periodically seeing the issue described below, but just now
figured out how to trigger it. It's sorta not a bug, I guess, but it
REALLY looks alarming to an unsuspecting user


1. Create a file t.el containing a single line
(message "\nDatabase directory: ")

2. emacs -Q t.el

3. C-s m (to isearch for "m")

4. M-s h r enter (to highlight "m" with the default face)

5. M-s h u

Step 5 should offer to unhighlight "m", but it offers garbage-looking
text instead. Upon closer inspection, it looks like the crazy garbage is
actually the regexp that char-folding produces when you look for "m". I
don't think this is what the user should see. As one consequence of
this, trying to unhighlight simply "m" doesn't work, even though that's
what the user asked to highlight in the first place.

Another possible side effect of this is that highlighting

  Database directory:

doesn't work: hi-lock goes through the motions but nothing ends up being
highlighted. Turning off char-folding fixes that.

Thanks!