Sync FSX fs_move code with latest FSFS changes.

* subversion/libsvn_fs_x/tree.c
  (is_changed_node): new utility
  (copy_helper): allow moving from older revisions if there were
                 no intermittent changes

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1527088 13f79535-47bb-0310-9956-ffa450edef68