patch 8.0.0956: scrolling in a terminal window has flicker

Problem:    Scrolling in a terminal hwindow as flicker when the Normal
            background differs from the terminal window background.
Solution:   Set the attribute to clear with.
diff --git a/src/message.c b/src/message.c
index 62e6bfe..57f33dd 100644
--- a/src/message.c
+++ b/src/message.c
@@ -2313,7 +2313,7 @@
 	gui_undraw_cursor();
 #endif
     /* scrolling up always works */
-    screen_del_lines(0, 0, 1, (int)Rows, TRUE, NULL);
+    screen_del_lines(0, 0, 1, (int)Rows, TRUE, 0, NULL);
 
     if (!can_clear((char_u *)" "))
     {
@@ -2905,7 +2905,7 @@
 		    }
 
 		    if (toscroll == -1 && screen_ins_lines(0, 0, 1,
-						       (int)Rows, NULL) == OK)
+						     (int)Rows, 0, NULL) == OK)
 		    {
 			/* display line at top */
 			(void)disp_sb_line(0, mp);