On the 'moves-tracking-2' branch: add a wrapper editor, intended more
as template code than to be useful in itself.

* subversion/libsvn_delta/editor3.c
  (wrapper_baton_t, wrap_*, svn_editor3_forwarding_wrapper): New.

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/move-tracking-2@1607421 13f79535-47bb-0310-9956-ffa450edef68