patch 8.0.1290: seq_cur of undotree() wrong after undo

Problem:    seq_cur of undotree() wrong after undo.
Solution:   Get the actual sequence number instead of decrementing the current
            one. (Ozaki Kiichi, closes #2319)
diff --git a/src/version.c b/src/version.c
index 41bfa74..949320c 100644
--- a/src/version.c
+++ b/src/version.c
@@ -762,6 +762,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1290,
+/**/
     1289,
 /**/
     1288,