bug#56393: Actually fix the long lines display bug

[Gregory Heytings]

> So you want to call the loop only when the buffer changed by two or more 
> characters?  Did you try something like
>
>  MODIFF > UNCHANGED_MODIFIED + 1
>
> ?  (This could be optimized further to use a margin larger that 1.)

That doesn't work alas, otherwise I would have used that of course. 
After C-u 100000 a, MODIFF - UNCHANGED_MODIFIED == 1 in a buffer without 
fontification and == 2 in a buffer with fontification, which does what it 
promises: it tells you that the buffer has been changed, but not how much.

I pushed an improved version of the heuristic, which uses both MODIFF / 
UNCHANGED_MODIFIED (to catch editing operations like M-% C-q C-j RET SPC 
RET) and the buffer size to decide whether a new detection should be 
performed.