Treat a partially visible line like "cursor off screen".

Treat a partially visible line like "cursor off screen".
(make_cursor_line_fully_visible): Don't try short scrolls.
Instead just return 0 when there is something to be done.
(try_scrolling): If make_cursor_line_fully_visible returns 0,
retry scrolling as if cursor were off the bottom.
(try_cursor_movement): If make_cursor_line_fully_visible returns 0,
return CURSOR_MOVEMENT_MUST_SCROLL.
(redisplay_window): If make_cursor_line_fully_visible returns 0,
go to try_to_scroll.