Follow-up to r1526057: don't call the generic FS API from FSFS but
out local implementation instead

* subversion/libsvn_fs_fs/tree.c
  (fs_closest_copy): forward declaration
  (is_changed_node): call FSFS implementation directly

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1526058 13f79535-47bb-0310-9956-ffa450edef68