Fix bug in gen_pktline() for deletes of missing remote refs

* gen_pktline() in smart_protocol.c was skipping refspecs that deleted
  refs that were not advertised by the server.  The new behavior is to
  send a delete command with an old-id of zero, which matches the behavior
  of the official git client.
* Update test_network_push__delete() in reaction to above fix.
* Obviate messy logic that handles missing push_spec rrefs by canonicalizing
  push_spec.  After calculate_work(), loid, roid, and rref, are filled in with
  exactly what is sent to the server