bug#56393: Actually fix the long lines display bug

[Gregory Heytings]

> And maybe this is not needed at all if restricting find_newline_no_quit 
> is enough to solve the problem.

Indeed, that's what I think.

> That's awesome, thanks.  I guess we will only need more measures if some 
> situations are reported where this is not fast enough, or causes some 
> other problems.

I don't think it's possible to make it faster.  I forgot to mention that 
C-s works instantly, too.

To anticipate one possible criticism: this feature comes at a (IMO) very 
little price, namely that inside long lines (and only inside long lines) 
line-move-visual does always not work as expected, point sometimes moves 
to another column (but in most cases does not).  That's the only 
compromise I did, and it is unavoidable.