improve multiple results support

The specification talks exactly about -1:
> Returns: the current result as an update count;
> -1 if the current result is a ResultSet object or there are no more results

Other negative numbers can theoretically mean something different than „no more results“
e.g. unknown number of updates (?).

#!/bin/bash

hg archive -r ${1} sql-dk_${1}_src.tar.gz