"git rebase --signoff" used to forget that it needs to add a
sign-off to the resulting commit when told to continue after a
conflict stops its operation.

* pw/rebase-m-signoff-fix:
  rebase -m: fix --signoff with conflicts
  sequencer: store commit message in private context
  sequencer: move current fixups to private context
  sequencer: start removing private fields from public API
  sequencer: always free "struct replay_opts"