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

[Stefan Monnier]

> As I said earlier, offering a way to escape the locked narrowing right away
> simply means that the current solution isn't one anymore.
> Elisp programmers would take the habit of using (widen-unlock) instead of
> (widen) in their programs, and in a couple of years we'll see again bug
> reports by users who cannot edit buffers with long lines.

I don't think we need such a paternalistic view of ELisp programmers.
ELisp programmers aren't out there looking for ways to mess things up.
If we give them good tools that make it easy to solve the usual problems
with needing `widen-unlock`, they won't start using it recklessly everywhere.


        Stefan