evolve: use the new rebased changeset to resolve phase-divergence
When there is a parent change in phase-divergence, `hg evolve` first rebases the
diveregent changeset on parent of public changeset and then continue the
resolution of phase-divergence *but* we used to continue with the old divergent
changeset rather than the new rebased one.
This patch makes sure we use the new rebased changeset to proceed with the
resolution. Tests changes shows that it fixes things which were broken earlier.