Implement a tree processor merge_dir_opened() in the merge code, by integrating
some code from the callback's merge_dir_added().

* subversion/libsvn_client/merge.c
  (merge_dir_opened): Implement the svn_diff_tree_processor_t api.
    Handle the existing node and add node cases by integrating the old ...
  (merge_dir_added): ... function, which is now empty.

  (ignore_dir_opened): New stub.
  (merge_callbacks): Use stub.
  (do_merge): Hook merge_dir_opened.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1440553 13f79535-47bb-0310-9956-ffa450edef68