rewind: use rewinded parent when creating multiple changesets Use use the rewinded parent instead of creating orphans based on identical, but obsolete, changesets.

rewind: add a test for rewinding a fold The code already have basic support for case. However the result create orphan that will need to be improved. However, we keep this changeset simple.

rewind: add a test for rewinding a split There are no code change needed but it is worth testing on its own.