 next() and ack().
  pack_data: Callback called for each bit of data in the pack
  progress: Callback for progress reports (strings)
  depth: Shallow fetch depth
  ref_prefix: List of prefixes of desired references, as a list of
    bytestrings. Filtering is done by the server if supported, and
    client side otherwise.
  filter_spec: A git-rev-list-style object filter spec, as bytestring.
    Only used if the server supports the Git protocol-v2 'filter'
    feature, and ignored otherwise.

Returns:
  FetchPackResult object

)