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

[Gregory Heytings]

> When 29 goes mainstream, just be prepared to auto-reply "Does your file 
> have a long line?" when feckless users ask, "Why did C-n go to a weird 
> column?"

We are prepared.

> And to be clear, you're going from "ineffable" performance to just 
> "underwhelming."  As of cb036a7 you still need find-file-literally for 
> some semblance of respectability.

Again your test case doesn't demonstrate anything useful, sorry.  You use 
a buffer with 100 lines, half of which are just one character above the 
threshold.  That's not at all representative, previous versions of Emacs 
(I tried 24, 25, 26, 27 and 28) can already edit such buffers fine.  Try 
the same with a buffer with 10 lines, each of which is 10 MB long.  Or 
with a buffer with a single 100 MB long line.