fix(up): divide the encoding of UploadPackReponse into two steps

[luckjay]

This PR is to divide the encoding of UploadPackReponse into two steps. So the user can use two different writers for pkt-lines and packfiles when integrating with side-band enabled clients, for example, isomorphic-git.

For example, the pseudo-code is like the following:

var w io.writer
if err = response.EncodePacketLines(w); err != nil {
	return error
}

muxer := sideband.NewMuxer(sideband.Sideband, w)
if err = response.EncodePackfiles(muxer); err != nil {
	return err
}

return nil

[luckjay]

@pjbgf could you review this pr, thanks!