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

[Eli Zaretskii]

> Cc: 56682@debbugs.gnu.org
> Date: Fri, 22 Jul 2022 18:06:44 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> 
> > The assertion below can now be violated:
> > 
> >       if (it->narrowed_begv)
> >     {
> >       record_unwind_protect (unwind_narrowed_begv, Fpoint_min ());
> >       record_unwind_protect (unwind_narrowed_zv, Fpoint_max ());
> >       SET_BUF_BEGV (current_buffer, it->narrowed_begv);
> >       SET_BUF_ZV (current_buffer, it->narrowed_zv);
> >       specbind (Qinhibit_widen, Qt);
> >     }
> > 
> >       val = Vfontification_functions;
> >       specbind (Qfontification_functions, Qnil);
> > 
> >       eassert (it->end_charpos == ZV);  <<<<<<<<<<<<<<<<<
> > 
> > because of the "narrowing".  (I actually saw this assertion violation
> > once on the branch, but I cannot reproduce it.)
> 
> Sorry, it's very easy to reproduce: visit dictionary.josn, and then
> type C-v several times until it happens.

I installed a fix.