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

[Gregory Heytings]

> > I see what you mean now.  But I don't think it would work.  What I want 
> > is to take reasonable measures to ensure that Emacs remains responsive 
> > "in spite of" mode maintainers.
>
> "Reasonable measures" is good.
>
> But once you step into "impossible to circumvent", you start going 
> directly against the goals of Free Software (and Emacs) to empower the 
> user.

But it is *not* impossible to circumvent.  Just add (setq 
long-line-threshold nil) in your init file, or evaluate it with M-:, or...

> I suffer from the same lack of imagination, as I sure many others here 
> do.  To make up for that, we've learned to follow a philosophy of 
> empowering the users (and not imposing arbitrary limits just because we 
> couldn't think of good reasons to go beyond them).

I'm all for empowering the users who want power, and the changeset is, 
intentionally, fully backward compatible.  In fact, strictly speaking, 
this changeset does exactly that: it empowers users, they can do things 
they couldn't do earlier.  But I'm also for protecting regular users who 
want a powerful editor that "just works".  In the same vein, we are 
marking a number of local variables as risky, to protect regular users, 
without preventing users from shooting themselves in the foot by adding

(defalias 'risky-local-variable-p 'ignore)

in their init file if they see fit.