When revisions are imported, we should attempt to preserve the parent revision from the source wiki. This means that if rev_id m has rev_parent_id n or 0 on the source wiki, then rev_id m' would have rev_parent_id n' or 0 on the target wiki, where the primes mean the corresponding imported revision IDs on the target wiki. If the parent ID on the source wiki is a deleted revision ID or has a different rev_page, then we would either have to fallback to using the preceding revision ID as rev_parent_id or insert dummy "ancestor" rows into the archive table.