patch 8.1.2289: after :diffsplit closing the window does not disable diff

Problem:    After :diffsplit closing the window does not disable diff.
Solution:   Add "closeoff" to 'diffopt' and add it to the default.
diff --git a/src/version.c b/src/version.c
index d2baa1f..205f2dc 100644
--- a/src/version.c
+++ b/src/version.c
@@ -742,6 +742,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    2289,
+/**/
     2288,
 /**/
     2287,