Simplified and improved heuristic for long lines detection.

Simplified and improved heuristic for long lines detection.

* src/lisp.h (modiff_incr): Add a parameter to 'modiff_incr' to record
the extent of the modification.

* src/insdel.c (insert_1_both, insert_from_string_1, insert_from_gap,
insert_from_buffer_1, adjust_after_replace, replace_range,
replace_range_2, del_range_2, modify_text): Add an argument to each
call to 'modiff_incr'.

* src/textprop.c (modify_text_properties): Add an argument to the call
to 'modiff_incr'.

* src/buffer.c (Frestore_buffer_modified_p, Fbuffer_swap_text,
modify_overlay): Add an argument to each call to 'modiff_incr'.

* src/xdisp.c (redisplay_window): Use the improved version of 'MODIFF'
for the heuristic.