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

[Gregory Heytings]

> I only care about the extra enforcement done in C code without providing 
> any mechanism to circumvent it.  Especially since this discussion seems 
> to suggest that if I were to propose a patch that makes this locking a 
> bit more "soft", it might be rejected on the grounds that it opens the 
> door to abuse, so not only I strongly dislike this design but I can't 
> even try and improve it.

I did not say that.  I do consider that it's not the right thing to do, 
for all the reasons I already gave, but if you have something concrete in 
mind, please show it.  I'm not the one who decides anyway.