Merge branch 'js/sequencer-and-root-commits'

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