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

[Eli Zaretskii]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: Gregory Heytings <gregory@heytings.org>,  56682@debbugs.gnu.org
> Date: Fri, 22 Jul 2022 19:25:47 -0400
> 
> For example, the behavior surprises me because I can't see why `M-x`
> should cause any font-locking at all in the `long-line.xml` file.

Any command that enters the minibuffer causes a thorough redisplay of
the windows on that frame (because more than one window has to be
updated).  What does that have to do with font-lock is a separate
question (but you are the best person to answer it).