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

[Stefan Monnier]

>> Just a curious question: How did we arrive at the current value of
>> `long-line-threshold'?
> I took the same value that Stefan had chosen for syntax-wholeline-max.

For the record, I chose 10K based on ... a wild guess that a 10K is
"definitely" in the camp of "very long lines" (it's not just a normal
line that happens to be a bit long) yet at the same time it is hopefully
short enough that it shouldn't take too much time to process.


        Stefan