In the commit harvester: make handle_descendants() use the standard
committables structure to detect and register committables.

* subversion/libsvn_client/commit_util.c
  (handle_descendants_baton): Add variable.
  (handle_descendants): Detect existing items via the hash table.
    Create items via add_committable().
  (svn_client__harvest_committables,
   harvest_copy_committables): Update caller.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1536914 13f79535-47bb-0310-9956-ffa450edef68