Clean up is_changed_node() implementation. No functional change.

* subversion/libsvn_fs_fs/tree.c
  (is_changed_node): reformat, add more commentary

Suggested by: ivan

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1527079 13f79535-47bb-0310-9956-ffa450edef68