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

[Gregory Heytings]

> > Hmmm...  After 350e97d78e, Isearch locks Emacs with 
> > toggle-truncate-lines. Recipe:
> >
> > C-x C-f long-line.xml
> > C-x x t
> > C-s </
> >
> > You have to kill Emacs, C-g does not work.
>
> I see the same on master, where that change is not yet installed.

Are you sure?  I don't see the same at 304e2a3a05.  C-s is slow, but it 
does not hang Emacs, and the effect of C-s </ is visible within a couple 
of seconds.