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

[Stefan Monnier]

> That's true, but with such big files, the initial scan is slow.  So the
> scenario is simple: you open a big enough file, type M->, and C-p.  M-> will
> be instantaneous, and C-p will take a while, because of syntax-ppss.

Really?  I'd expect that `M->` is slow because of `syntax-ppss`
(called by font-lock) and then `C-p` is instantaneous.


        Stefan