Switch the last merge function that determined obstructions itself over to the
common obstruction/tree conflict code.

* subversion/libsvn_client/merge.c
  (merge_dir_added): Simplify code by using perform_obstruction_check().


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1433715 13f79535-47bb-0310-9956-ffa450edef68