evolve: move logic to resolve phase-divergence in a new function This patch moves logic which does the revert and create a commit on top of public changeset because we need to repeat that logic while handling continuation of `hg evolve` when there are conflicts while rebasing the phase-divergent changeset.

evolve: store the precursor of phase-divergent in evolvestate This patch makes `hg evolve` store the precursor of phase-divergent changeset in evolvestate so that we don't need to calculate that again.

evolve: move logic to complete interrupted relocation to new fn This moves logic which completed the interrupted relocation to it's own separate function as we will need that function in continuing phase-divergence and content-divergence too.