patch 9.0.0539: long message test can be flaky

Problem:    Long message test can be flaky.
Solution:   Wait for more prompt instead of ruler.
diff --git a/src/version.c b/src/version.c
index 0bbeae6..359c70c 100644
--- a/src/version.c
+++ b/src/version.c
@@ -700,6 +700,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    539,
+/**/
     538,
 /**/
     537,