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)

src/version.c

@@ -761,6 +761,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1290,
 /**/
     1289,
 /**/