Minor additional fix in the wake of bugs #13623 and 13626.

Minor additional fix in the wake of bugs #13623 and 13626.

 src/xdisp.c (redisplay_internal): Don't set w->region_showing to the
 marker's position.
 (display_line): Set w->region_showing to the value of
 it->region_beg_charpos, not to -1.  This fixes redisplay
 optimization when cursor is moved up after M->.
 src/window.h (struct window): region_showing can no longer be negative.