bug#57669: 29.0.50; C-n, C-p off under long lines

[Gregory Heytings]

> > Under long-line-threshold=10000 find-file 99s
>
> This means on average 1 sec per C-n.  Which is tolerable.

The buffer contains ten lines each of which is 100K long, so C-n is called 
much more than 100 times, it is called ~10000 times.  So that's actually 
~10 ms per C-n.