evolve: introduce a resolutionparent variable and store it in evolvestate
This patch introduces a resolutionparent variable which will be the parent of
final changeset after resolving content-divergent changesets.
Till now, the parent is always the parent of one of the divergent node, but now
in upcoming cases, it can be a successor of both the parents of divergent nodes.
test-drop.t
test-inhibit.t
test-simple4server.t
tests/test-simple4server-bundle2.t