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

[Gregory Heytings]

> > In this case, "arbitrarily large" contradicts the text that follows, 
> > which describes the circumstances where that might not be true.
>
> I've carefully chosen the words of the title, and it doesn't contradict 
> what follows, as far as I understand.  It says "Emacs is now capable of 
> editing files with arbitrarily long lines", in which "capable" means 
> that it can do it, but will not always do it.  The circumstances that 
> are described in the text that follows tell the reader that the 
> remaining cases in which Emacs would choke on such files are outside of 
> Emacs' responsibility, they are the responsibility of major and minor 
> mode writers.

I see that you've already reverted.  Sigh.