bundle2: comment to clarify why the handler call is where it is The reason why it is here is not obvious. I'm the one who wrote it there in the first place and almost moved it 2 weeks later.

bundle2: use chunkbuffer for exchange.getbundle We can use `util.chunkbuffer` instead.

bundle2: lazily generate the changegroup part in exchange.getbundle Now that we have lazy generation of parts, let's use it.

bundle2: support chunk iterator as part data When the `part.data` attribute is an iterator, we assume it is an iterator of chunks and use it. We use a chunkbuffer to yield chunks of 4096 bytes. The tests are updated to use this feature.

bundle2: extract a _payloadchunks method for part We are preparing streaming capability for part. So the generation of payload chunk will becomes more complex. We extract this part in its own function before any changes.

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.