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.

bundle2: add reply awareness to unbundlerecords We need an efficient way to handle bundle replies. The unbundle records class is extended to carry such data.

bundle2: add an integer id to part For sending response to a pushed bundle, we need to link reply parts to request part. We introduce a part id for this purpose. This is a 32 bit unique integer stored in the header.