bug#56682: Fix the long lines font locking related slowdowns

[Eli Zaretskii <eliz@gnu.org>]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: dgutov@yandex.ru,  56682@debbugs.gnu.org,  gregory@heytings.org
> Date: Tue, 16 Aug 2022 13:15:04 -0400
> 
> Eli Zaretskii [2022-08-16 18:51:41] wrote:
> >> I missed it then: you're saying that I can make nlinum-mode override the
> >> locked narrowing to find the real current line number without having to
> >> recompile Emacs?
> >
> > No, I'm saying that you can test improvements in nlinum-mode that
> > would make it work better with long lines without recompiling.
> 
> How?

I'm sure you can figure it out if you just think for a few seconds.

But if you want me to do it for you, then please tell which changes do
you want to try and in what situations, because the technique depends
on that.

(Not that I understand where are you going with this.  Especially
since a feature to unlock the narrowing is just one simple commit
away.)