- Improved support for lightweight tags
  - Supports create/update/delete
  - Show only current ref pointed to (or previously
    pointed to)

- Show previous value of ref when deleted
  - Useful for recovering from accidental deletions

- Distinguish between rewind and rebase
  - Show only new branch head on rewind
  - Show all commits on rebase if only commit message(s)
    changed (otherwise we wouldn't know about the commit
    at all)

- Minor cleanup (remove duplicate code, etc.)