evolve: fix stabilization of orphan merge changesets in case of conflicts
As demonstrated by tests added in previous patch, if one of the parent of a
merge changeset has a new version and merge changeset gets, running `hg evolve`
to stabilize that if results in conflicts will lead to lose of other parent of
that merge changeset when we resolve conflicts and do `hg evolve --continue`.
This patch fixes the behavior, by checking whether we are continuing evolving a
merge changeset and if yes, setting the dirstate parents according to that.