The implementation of "git rebase -i --root" has been updated to use
the sequencer machinery more.

* js/sequencer-and-root-commits:
  rebase --rebase-merges: root commits can be cousins, too
  rebase --rebase-merges: a "merge" into a new root is a fast-forward
  sequencer: allow introducing new root commits
  rebase -i --root: let the sequencer handle even the initial part
  sequencer: learn about the special "fake root commit" handling
  sequencer: extract helper to update active_cache_tree