bug#56393: Actually fix the long lines display bug

[Gregory Heytings]

> Do you need it to run as often as the current code does?  It's a 
> heuristic -- if it gets it wrong in some pathological situation (i.e., 
> if the user replaces all newlines with spaces in a huge buffer with a 
> function that does just that), that's not a deal breaker.

I think you need, yes.  And with the present heuristic it will not run 
that often, in fact.  The problem is that a single editing operation like 
C-u 100000 a creates a long line, and Emacs would start choking again. 
Or C-x i <a file with long lines>.  Or a shell process which produces a 
long line.  And so forth.

> That is, I don't think you have to run the detection as often as you do 
> now.  If the previous call said you had no lines longer than 80 
> characters, you don't have to run it again until the buffer has grown 
> more than 30K characters, or you have a modiff in the thousands region.

Hmmm...  Thanks, I'll see if I can do something with that idea.