patch 9.0.1686: undotree() only works for the current buffer

Problem:    undotree() only works for the current buffer
Solution:   Add an optional "buffer number" parameter to undotree().  If
            omitted, use the current buffer for backwards compatibility.

closes: #4001
closes: #12292

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: zeertzjq <zeertzjq@outlook.com>
Co-authored-by: Devin J. Pohly <djpohly@gmail.com>
diff --git a/src/version.c b/src/version.c
index 7134218..f4c73e7 100644
--- a/src/version.c
+++ b/src/version.c
@@ -696,6 +696,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1686,
+/**/
     1685,
 /**/
     1684,