run-tests: Pass arguments into argument parser Before, arguments were not passed into the optparse.OptionParser instance and were coming from sys.argv. This patch enables consumers to define the list of arguments to parse without having to adjust sys.argv.

bundle2: rename part to bundlepart We are going to introduce an `unbundlepart` dedicated to reading bundle. So we need to rename the one used to create bundle. Even if dedicated to creation, this is still used for unbundling until we get the new class.

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.

bundle2: use reply part to return result of addchangegroup We now have an official way to return the result of addchangegroup. The tests are updated to check that the return bundle is properly created. It will be used when push is bundle2 enabled.

bundle2: produce a bundle2 reply We do not know yet what kind of data future features and extensions will need to exchange. To handle that, bundle2 allows to send arbitrary content to the server. As a consequence, we need to be able to reply arbitrary content to the client. And, we can use bundle2 to transmit those arbitrary data. When a client will push a bundle2 to the server, the server will reply with a bundle2 itself. This changeset installs the first stone of this logic and test it.