patch 9.0.0307: :echomsg doesn't work properly with cmdheight=0

Problem:    :echomsg doesn't work properly with cmdheight=0.
Solution:   Improve scrolling and displaying.
diff --git a/src/eval.c b/src/eval.c
index 3d6d84c..8cd2716 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -6824,7 +6824,18 @@
 
     if (eap->skip)
 	--emsg_skip;
+#ifdef HAS_MESSAGE_WINDOW
+    if (use_message_window() && eap->cmdidx != CMD_execute)
+    {
+	// show the message window now
+	ex_redraw(eap);
 
+	// do not overwrite messages
+	msg_didout = TRUE;
+	if (msg_col == 0)
+	    msg_col = 1;
+    }
+#endif
     set_nextcmd(eap, arg);
 }