Merge branch 'aw/rebase-i-edit-todo'

Merge branch 'aw/rebase-i-edit-todo'

Teach an option to edit the insn sheet to "git rebase -i".

* aw/rebase-i-edit-todo:
  rebase -i: suggest using --edit-todo to fix an unknown instruction
  rebase -i: Add tests for "--edit-todo"
  rebase -i: Teach "--edit-todo" action
  rebase -i: Refactor help messages for todo file
  rebase usage: subcommands can not be combined with -i