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

[Stefan Monnier]

> Indeed, that's my conclusion too.  So until syntax-ppss (at least) is made
> an order of magnitude faster, the right thing to do is to use the forced
> narrowing method.

By tying this to "long lines" is wrong, since it has nothing to do with
long lines, only with large buffers.


        Stefan