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

[Gregory Heytings]

> > > > Feel free to enlarge long-line-threshold in your init file.
> > >
> > > Just a curious question: How did we arrive at the current value of 
> > > `long-line-threshold'?
> >
> > I took the same value that Stefan had chosen for syntax-wholeline-max.
>
> And it isn't sacred in any way.  If we decide a different value strikes 
> a better balance, we will change it.

Indeed (although I fear a bikeshedding here).