patch 9.2.0591: 'scrolljump' ignored when scrolling up

patch 9.2.0591: 'scrolljump' ignored when scrolling up

Problem:  srolljump=-100 only scrolls half a page going up, but works
          fine going down. update_topline() always falls back to
          scroll_cursor_halfway() when the cursor is far above topline.
Solution: Only center when sj is smaller than half the window. Otherwise
          call scroll_cursor_top like the downward path does (glepnir).

fixes:  #1527
closes: #20366

Signed-off-by: glepnir <glephunter@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>