parsers: optimize filtered headrevs logic The old native head revs logic would iterate over every node, starting from 0, and check if every node was filtered (by testing it against the filteredrevs python set). On large repos with hundreds of thousands of commits, this could take 150ms. This new logic iterates over the nodes in reverse order, and skips the filtered check if we've seen an unfiltered child of the node. This saves approximately a bagillion filteredrevs set checks, which shaves the time down from 150ms to 20ms during every branch cache write.