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

[Gregory Heytings]

> > > > There have been many more complaints about being unable to edit a 
> > > > file with a 5000+ line comfortably, than about editing 100+ MB files.
> > >
> > > Also because we have things like so-long to handle the large file 
> > > case.
> >
> > These things are expedients for want of anything better, aren't they?
>
> Yes, just like the narrowing your use in your long lines patch.

That's... unfair.  These things turn off everything (in particular, the 
major mode and font locking) and provide a bare-bones editing experience. 
What we'd like now is to provide Emacs users a near-complete editing 
experience in all buffers.

> They're slightly different in that they don't try to prevent replacing 
> those solutions with something better (or worse), tho.

When one looks at things from far enough, everything is indeed "slightly 
different" from everything else.  For example, the Statue of Liberty is 
"slightly different" from Empire State Building.