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

[Gregory Heytings]

> > > You can maybe have that for C-p that follows M->, but wouldn't the 
> > > wait return, with a vengeance, if you insert a single character 
> > > (because then the buffer needs to be re-scanned)?  If so, we've gained 
> > > nothing, really.
> >
> > Fortunately no: the buffer doesn't need to be rescanned, syntax-ppss 
> > caches its result, to avoid having to rescan the whole buffer again and 
> > again.
>
> But the buffer has changed, so the cache is not necessarily valid, 
> right?

I didn't look at the internals of syntax-ppss, but I'd guess (and again my 
tests seem to confirm) that it was designed well enough, and doesn't need 
to rescan the whole buffer whenever a single character is inserted.