patch 9.0.0644: 'smoothscroll' is not copied to a new window on :split

Problem:    'smoothscroll' is not copied to a new window on :split.
Solution:   Copy the option value.  Add a test.
diff --git a/src/version.c b/src/version.c
index a228d74..32a9851 100644
--- a/src/version.c
+++ b/src/version.c
@@ -700,6 +700,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    644,
+/**/
     643,
 /**/
     642,