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

[Gregory Heytings]

> And another thing: the condition on it->narrowed_begv being non-zero 
> means that as long as we are close enough to the beginning of a buffer, 
> we don't restrict fontification-functions from going as far as they want 
> into the buffer.
>
> So I think the condition should be the long_line_optimizations_p flag of 
> the buffer, and we should narrow the buffer even when we are at BOB, to 
> prevent fontification-functions from going too far.

That's correct, indeed, thanks.  Now fixed on master.