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

[Eli Zaretskii]

> Date: Tue, 26 Jul 2022 12:34:30 +0000
> From: Gregory Heytings <gregory@heytings.org>
> cc: gerd.moellmann@gmail.com, 56682@debbugs.gnu.org, monnier@iro.umontreal.ca
> 
> > I don't know the difference between "slow" and "hang".  Gerd says it 
> > eventually finishes, so it isn't a "hang" at least on his machine. And 
> > my build is unoptimized, so what is "slow" for you probably qualifies as 
> > "hang" for me.
> 
> I just tried again, and on my machine with an optimized build C-s </ RET 
> takes 220 seconds to finish (and cannot be interrupted).  That's what I 
> call "hang".  With 350e97d78e, C-s </ RET takes about 1 second.  That's 
> what I call "slow" (without truncate-lines C-s </ RET is instantaneous).

OK, I will take a look soon.  That change makes us use a branch of
code that AFAIU was never used since it was written, so some kind of
trouble can be expected.