changegroup: move chunk extraction into a getchunks method of unbundle10 This code used to be in `writebundle` only. We needs to make it more broadly available for bundle2. The "changegroup" bundle2 part has to retrieve the binary content of changegroup stream. We moved the chunks retrieving code into the `unbundle10` object directly and the `writebundle` code is now using that. This split is useful for bundle2 purpose, we want to be able to easily stream changegroup content in a part. To keep thing simples, we kept compression out of the new methods. If it make more sense in the future, compression may get included in this function too.