bug#56393: Actually fix the long lines display bug

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: 56393@debbugs.gnu.org,  Eli Zaretskii <eliz@gnu.org>,  Gerd
>  Möllmann
>  <gerd.moellmann@gmail.com>
> Date: Wed, 06 Jul 2022 13:31:47 +0200
> 
> There's some slight hangs when `C-s'-ing the file, but none that last
> longer than a second or two.  (Perhaps that has something to do with
> `C-s' trying to add highlights to all the matches?)

If this was in an optimized build, it is worth investigating, IMO:
second or two is quite long for an optimized build.