In the wc-repos diff wrap the callbacks with a diff-tree processor.

This allows refactoring the code without changing the public api yet.

* subversion/libsvn_wc/diff_editor.c
  (edit_baton): Add diff processor.
  (make_edit_baton): Initialize diff processor.
  (file_diff): Drive the diff processor in two cases, instead of calling
    the callback apis directly.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1444256 13f79535-47bb-0310-9956-ffa450edef68