Switch the two most trivial merge-diff-callbacks over to the diff processor.

* subversion/libsvn_client/merge.c
  (merge_file_opened): Implement svn_diff_tree_processor_t callback type.
  (merge_file_changed): Implement svn_diff_tree_processor_t callback type.
    Handle argument renames.

  (ignore_file_opened,
   ignore_file_changed): New stub function.
  (merge_callbacks): Update references.
  (do_merge): Hook file opened and changed to the diff processor.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1440442 13f79535-47bb-0310-9956-ffa450edef68