Further tweaks to long lines handling.

Further tweaks to long lines handling.

* src/xdisp.c (redisplay_window): Increase the threshold above which
long lines detection is performed in the buffer.  This should avoid
triggering that detection for most simple editing operations.

* src/lisp.h (modiff_incr): Explain why the counter is incremented
logarithmically.

* src/buffer.h (struct buffer_text): Adapt the comment about the
'modiff' field accordingly.

* src/buffer.c (modify_overlay): Increase the counter by 1 instead of
the size of the buffer section on which the overlay is placed.

* etc/NEWS: Small improvement.