bug#56393: Actually fix the long lines display bug

[Gregory Heytings]

> That's not the problem you presented.  You presented an opposite 
> problem: where inserting many characters bumps the tick by just 1.  I 
> asked why that is a problem.

We are miscommunicating.  I was not telling you that the fact that 
inserting many characters bumps the tick by just 1 is in itself a problem. 
I was telling you that it makes that tick unusable for the present task.

> > Fine, but then you'd have to tell me how to do that in another way 
> > (that is, how to not trigger the long line detection code when the 
> > buffer has only changed a little), or to convince me that the 13 ms 
> > overhead is okay.
>
> That's not what I'm saying.  I'm saying that not recomputing the long 
> lines when the user types "C-u 100000 a" is not a catastrophe.

It is, at least if this is meant to be a full and complete solution to 
that problem, because the same happens for example with C-x i <a file with 
long lines>: the tick does not change "enough", long line optimizations 
would not be enabled, and Emacs would choke again.  Or if a user 
innocently types "cat <a file with long lines>" in a shell buffer without 
knowing what that file contains.