As final step of dealing with the holes in the phase numbers, make
phaseroots and phasesets both dictionaries indexed by the phase number.
Further adjust the interface of the C module by pushing the node to
revision mapping down as it is cheaper on the C side to deal with
Overall, the patch series improves a no-change "hg up" for my NetBSD test
repository from 4.7s to 1.3s.