patch 9.0.0114: the command line takes up space even when not used

Problem:    The command line takes up space even when not used.
Solution:   Allow for 'cmdheight' to be set to zero. (Shougo Matsushita,
            closes #10675, closes #940)
diff --git a/src/ops.c b/src/ops.c
index 3a77164..c160e88 100644
--- a/src/ops.c
+++ b/src/ops.c
@@ -3260,6 +3260,11 @@
 	    // Don't shorten this message, the user asked for it.
 	    p = p_shm;
 	    p_shm = (char_u *)"";
+	    if (p_ch < 1)
+	    {
+		msg_start();
+		msg_scroll = TRUE;
+	    }
 	    msg((char *)IObuff);
 	    p_shm = p;
 	}