patch 8.0.0837: signs can be drawn on top of console messages

Problem:    Signs can be drawn on top of console messages.
Solution:   don't redraw at a prompt or when scrolled up. (Christian Brabandt,
            closes #1907)
diff --git a/src/screen.c b/src/screen.c
index ea07d02..77d0e4f 100644
--- a/src/screen.c
+++ b/src/screen.c
@@ -1019,8 +1019,11 @@
     }
 
     /* Return when there is nothing to do, screen updating is already
-     * happening (recursive call) or still starting up. */
+     * happening (recursive call), messages on the screen or still starting up.
+     */
     if (!doit || updating_screen
+	    || State == ASKMORE || State == HITRETURN
+	    || msg_scrolled
 #ifdef FEAT_GUI
 	    || gui.starting
 #endif