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

[Gregory Heytings]

> The usual interpretation of "capable" in Emacs is that we do it unless 
> the user tells us not to.  Otherwise users will ask why not do it 
> whenever possible, definitely for a feature like this one.  So that is 
> the contradiction which I had in mind.

I understand.  Nonetheless, the OED tells me "capable: adj. 1 (capable of 
doing something) having the ability or quality necessary to do something."