bug#56393: Actually fix the long lines display bug

[Gregory Heytings]

> Why just "added"?  Deletion of characters can also create long lines, if 
> the deleted characters are newlines.

Indeed, somehow I missed that possibility.  I think a better heuristic 
would be: if more than one character was added or removed from the buffer, 
check whether the buffer contains long lines.  I want to avoid affecting 
normal typing as much as possible, even if the effect is small.

> Also, what if since last redisplay N characters were deleted and N other 
> characters were added?

You mean for example M-% C-j RET SPC RET?  Indeed, that's yet another 
possibility.

> And that's on top of the main reason: BUF_CHARS_MODIFF exists for a long 
> time, so introducing yet another similar counter sounds like not the 
> best idea to me.

To be honest, I don't really understand the purpose of BUF_CHARS_MODIFF. 
As the comment in buffer.h explains: "It is set to modiff for each such 
event, and never otherwise changed."  So it doesn't contain new 
information.  And it's inconvenient that you have to keep a copy of its 
previous value around, using MODIFIED != UNCHANGED_MODIFIED as I did 
earlier is much easier.