patch 9.0.0316: screen flickers when 'cmdheight' is zero

Problem:    Screen flickers when 'cmdheight' is zero.
Solution:   Redraw over existing text instead of clearing.
diff --git a/src/ex_getln.c b/src/ex_getln.c
index 58dc152..f28ce8a 100644
--- a/src/ex_getln.c
+++ b/src/ex_getln.c
@@ -2582,7 +2582,7 @@
 	    made_cmdheight_nonzero = TRUE;
 	    set_option_value((char_u *)"ch", 0L, NULL, 0);
 	    // Redraw is needed for command line completion
-	    redraw_all_later(UPD_CLEAR);
+	    redraw_all_later(UPD_NOT_VALID);
 	    made_cmdheight_nonzero = FALSE;
 	}