Improve detection of long lines.

Improve detection of long lines.

* src/buffer.h (struct buffer_text): New field.
(BUF_CHARS_UNCHANGED_MODIFIED, CHARS_UNCHANGED_MODIFIED): New macros.

* src/buffer.c (Fget_buffer_create): Initialize the new field.

* src/xdisp.c (mark_window_display_accurate_1): Set the new field.
(redisplay_window): Use it, together with CHARS_MODIFF, instead of
MODIFF and UNCHANGED_MODIFIED to decide whether to check for long
lines.

* src/pdumper.c (dump_buffer): Dump the new field.