code: protected final String groupKey(CDXLine line) {
  		String timestamp = line.getTimestamp();
  		return timestamp.substring(0,
  			Math.min(timestampDedupLength, timestamp.length()));
  	}
docs: return group key of {@code line}.
  @param line CDX line
  @return first {@code timestampDedupLength} digits
  of {@code timestamp}