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.