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

[Gregory Heytings]

> > It is, indeed.  But here we are talking about big files (say 1 GB). 
> > In that case (and in that case only) it is much better, from a user 
> > viewpoint, to wait say 20 seconds before the file is opened and being 
> > at that point able to freely move through the file, instead of waiting 
> > only 6 seconds, and then having to wait another 10 seconds between two 
> > motion commands like M-> C-p.  In fact, no user expects that a 1 GB 
> > file would open instantaneously.
>
> 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.  At least that's what Stefan and Dmitry told me, and that's what I 
see in my tests.