Merge branch 'js/sequencer-i-countdown-3'

The sequencer machinery has been further enhanced so that a later
set of patches can start using it to reimplement "rebase -i".

* js/sequencer-i-countdown-3: (38 commits)
  sequencer (rebase -i): write out the final message
  sequencer (rebase -i): write the progress into files
  sequencer (rebase -i): show the progress
  sequencer (rebase -i): suggest --edit-todo upon unknown command
  sequencer (rebase -i): show only failed cherry-picks' output
  sequencer (rebase -i): show only failed `git commit`'s output
  sequencer: use run_command() directly
  sequencer: update reading author-script
  sequencer (rebase -i): differentiate between comments and 'noop'
  sequencer (rebase -i): implement the 'drop' command
  sequencer (rebase -i): allow rescheduling commands
  sequencer (rebase -i): respect strategy/strategy_opts settings
  sequencer (rebase -i): respect the rebase.autostash setting
  sequencer (rebase -i): run the post-rewrite hook, if needed
  sequencer (rebase -i): record interrupted commits in rewritten, too
  sequencer (rebase -i): copy commit notes at end
  sequencer (rebase -i): set the reflog message consistently
  sequencer (rebase -i): refactor setting the reflog message
  sequencer (rebase -i): allow fast-forwarding for edit/reword
  sequencer (rebase -i): implement the 'reword' command
  ...