In svnpredumpfilter.py, store copies as a set of tuples instead of as a dict.

* tools/server-side/svnpredumpfilter.py
  (handle_changes): Iterate the set.
  (svn_log_stream_get_dependencies): Create path_copies a set and use the
   set's add() method to add copy-tuples.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1625709 13f79535-47bb-0310-9956-ffa450edef68