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

[Juri Linkov <juri@linkov.net>]

> 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.

Unlike bug#21180, it's much harder to conceal the innards of hi-lock.
Its regexps will keep pouring in from all places.

> 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.

Actually “Database directory:” is not highlighted due to case-folding.
After toggling case-folding with ‘M-s c’ and preserving the capital D,
it's highlighted correctly.