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

[Gregory Heytings]

> > > The locked narrowing currently in effect makes it impossible to 
> > > investigate this problem or improve the behavior :-(
> >
> > How so?  It is fully backward compatible, just (setq 
> > long-line-threshold nil).  If what you want is only to remove the 
> > locked narrowing, comment out the relevant line in xdisp.c.
>
> That brings back the performance problems in redisplay, which have a 
> more pronounced effect, overshadowing both issues and potential 
> improvements in font-lock.

The first option does, the second does not.