rewind: move revision selection into its own function This will help making the logic more advanced.

rewind: automatically rewind entire stack We now rewind the full stack, avoiding creating orphans. A `--exact` flag is added to force rewinding only the explicitly specified changesets.

rewind: add a test about rewinding top of stack For now this rewind is creating an orphan. We add the test to check the behavior in that case.

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.

rewind: update the working copy if it gets obsoleted If the working copy parent is rewinded, follow the rewind.

rewind: add a message about obsolete changeset Not the best output, but useful to have the data. We can improve that later.

rewind: obsolete latest successors unless instructed otherwise To prevent the creations of divergence, we automatically mark the successors set of the rewinded changeset as obsoleted by the rewind result. A new flag `--as-divergence` is added to disable this behavior.

rewind: test rewinding an amended changesets We are now able to rewind a simple amended changeset. For now, its current successors has not been obsoleted and we created divergence.