Miscellaneous cleanup.

- Fail if --message flag is provided with update subcommand
- Add more general function get_change_id_from_commit in addition to
  get_change_id_from_head
- Rename get_branch to the more apt get_current_branch
- Clean up naming, comments and doc strings

Change-Id: Id966133b2bb0e66e1902bcee42303b70033744f7