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/version.c b/src/version.c
index 94a1701..2d02461 100644
--- a/src/version.c
+++ b/src/version.c
@@ -790,6 +790,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    135,
+/**/
     134,
 /**/
     133,