Following up on r1438261, implement a diff tree processor that wraps the old
diff callback layer. This private api also includes an additional callback
for handling the state output arguments that are used only by libsvn_client's
internal code.

* subversion/include/private/svn_diff_tree.h
  (svn_diff_source_t): Uncomment optional components.

* subversion/include/private/svn_wc_private.h
  (includes): Add svn_diff_tree.h.
  (svn_wc__diff_state_handle_t,
   svn_wc__diff_state_close_t): New typedef.
  (svn_wc__wrap_diff_callbacks): New function.

* subversion/libsvn_wc/diff_editor.c
  (includes): Add svn_diff_tree.h.
  (wc_diff_wrap_baton_t): New struct.
  (wrap_*): New functions.
  (svn_wc__wrap_diff_callbacks): New function.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1438264 13f79535-47bb-0310-9956-ffa450edef68