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

[Eli Zaretskii]

> Date: Thu, 21 Jul 2022 18:00:37 +0000
> From: Gregory Heytings <gregory@heytings.org>
> 
> New bug number and thread to discuss the next chapter of the long lines 
> slowdowns.

Thanks.

FTR, I repeat here the recipe from
https://debbugs.gnu.org/cgi/bugreport.cgi?bug=56393#425:

  emacs -Q
  C-x C-f long-line.xml RET

Now, do NOT disable font-lock, and wait for Emacs to say "Valid" in
the mode line (to get nXML mode out of the way).  Then:

  M-x toggle-truncate-lines RET

Now simple cursor motion commands that use redisplay optimizations are
fast, but commands that cause more thorough redisplay are as slow as
on master.  As a simple example, try just "M-x" and wait until the
"M-x" prompt appears in the minibuffer -- here it takes much longer,
basically as long as the version on master.