Improve the heuristic for long lines detection.

Improve the heuristic for long lines detection.

* src/buffer.h (struct buffer_text): New 'unchanged_size' field.
(BUF_UNCHANGED_SIZE): New macro to access the field.

* src/buffer.c (Fget_buffer_create): Initialize the field.
(Fbuffer_swap_text): Handle it.

* src/xdisp.c (mark_window_display_accurate_1): Set the field.
(redisplay_window): Use the field for long lines detection.