Follow-up to r1527088.

* subversion/libsvn_fs_x/tree.c
  (is_changed_node,
   copy_helper): call FSX internal API instead of FSFS's interna

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1527091 13f79535-47bb-0310-9956-ffa450edef68