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

[Eli Zaretskii]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: gregory@heytings.org,  dgutov@yandex.ru,  56682@debbugs.gnu.org
> Date: Fri, 05 Aug 2022 08:06:32 -0400
> 
> > If and when it is solved, we could revisit this issue.
> 
> The locked narrowing currently in effect makes it impossible to
> investigate this problem or improve the behavior :-(

??? Just set long-line-threshold to nil, and you can investigate all
you want.