bug#56683: 29.0.50; long lines fix doesn't work correctly when lines are truncated

[Gregory Heytings]

> Overall, I'd like not to lose the ability to view long lines in such 
> logs with truncated lines enabled, because as I've said earlier, it 
> allows me to partially compare line to above and below ones, without 
> scrolling. And I rarely even scroll to the right that much in such logs, 
> as there's just way too much information to be viewed in a single 
> horizontal line.

As far as I understand, for your use case disabling truncate-lines when 
the buffer contains long lines would not change anything: you said that in 
your case the long lines are never longer than 10000 characters, which is 
the (default) limit of long-line-threshold.  Should you ever encounter a 
slightly longer line, you could always adapt long-line-threshold 
accordingly.  The point is that there's no way to make truncate-lines work 
with arbitrarily long lines, say a 1 GB long line.

(A side note: if you rarely scroll to the right that much, you can also 
truncate the lines with an external tool like "cut", either before you 
open the file in Emacs, or inside Emacs with C-x h M-| cut -c-1000 RET.)