message: Describe game_id transformation
diff: @@ -150,10 +150,15 @@ void PortGamesToBigtable(const std::string& gcp_project_name,
  auto row_prefix = absl::StrFormat("g_%010d", game_counter);
  + // Transforms something like:
  + // gs://minigo/data/play/2018-10-14-13/1539522000-8x7lb.tfrecord.zz
  + // into:
  + // 2018-10-14-13-1539522000-8x7lb
  auto game_id = path;
  game_id.erase(game_id.rfind(".tfrecord.zz"));
  game_id[game_id.rfind('/')] = '-';
  game_id.erase(0, game_id.rfind('/') + 1);
  +
  auto zero_row = absl::StrFormat("%s_m_%06d", row_prefix, 0);
  google::cloud::bigtable::SingleRowMutation zero_row_mutation(zero_row);
  zero_row_mutation.emplace_back(SetCell("metadata", "game_id", game_id));
  