tools/batch-rebase: Allow adding tags between topics:

topics:
  -
    name: foo
    remote: bar
    base: foo-base
    tip: foo

 # Add a tag between topics, to mark a boundary
  -
    action: tag
    name: mytag
    # Empty string to avoid the suffix given with --tags-suffix
    suffix: ""

  -
    name: foo2
    remote: bar
    base: foo-base2
    tip: foo2