phases: sparsify phaseroots and phasesets 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 revision numbers. Overall, the patch series improves a no-change "hg up" for my NetBSD test repository from 4.7s to 1.3s. Differential Revision: https://phab.mercurial-scm.org/D8698