On the 'move-tracking-2' branch: Don't crash when deleting a tree.

* subversion/libsvn_delta/compat3.c
  (branch_map_purge_orphans): New.
  (svn_branch_instance_serialize,
   convert_branch_to_paths): Use it.

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/move-tracking-2@1634811 13f79535-47bb-0310-9956-ffa450edef68