patch 8.1.0135: undo message delays screen update for CTRL-O u

Problem:    Undo message delays screen update for CTRL-O u.
Solution:   Add smsg_attr_keep(). (closes #3125)
diff --git a/src/undo.c b/src/undo.c
index 117321e..be4e22e 100644
--- a/src/undo.c
+++ b/src/undo.c
@@ -2968,7 +2968,7 @@
     }
 #endif
 
-    smsg((char_u *)_("%ld %s; %s #%ld  %s"),
+    smsg_attr_keep(0, (char_u *)_("%ld %s; %s #%ld  %s"),
 	    u_oldcount < 0 ? -u_oldcount : u_oldcount,
 	    _(msgstr),
 	    did_undo ? _("before") : _("after"),