bundle2: allow bundle2 for pulling over the wire This changeset makes `wireprotocol` peers advertise bundle2 capability and comply with bundle2 `getbundle` requests. Note that advertising bundle2 could make a client try to use it for push. Such pushes would fail. However, I do not expect any human being to have enabled bundle2 on their server yet.

bundle2: return a stream from exchange.getbundle For friendliness with the wire protocol implementation, the `exchange.getbundle` now returns a binary stream when asked for a bundle2. We detect a bundle2 request and upgrade the binary stream to an unbundler object. In the future the unbundler may gain feature to look like a binary stream, but we are not quite there yet.

bundle2: let readbundle return unbundle20 The `readbundle` function can now recognize a bundle2 stream and return the appropriate unbundler. This is required for proper bundle2 support over the wire.

bundle2: make header reading optional The `readbundle` function will consume the 4 first bytes to dispatch between various unbundler. We introduce a way to inform `unbundle20` that the header has been read and it can be trusted.

bundle2: prepare readbundle to return more that one type of bundle We first read 4 bytes to get the `HG10` bytes then we read the compression scheme if this is `HG10`. This prepares the code for the arrival of `HG20` handling.

bundle2: add a ui argument to readbundle The bundle2 unbundler needs a ui argument. We are now passing this information to `readbundle`.

bundle2: move `readbundle` into the `exchange` module The `readbundle` function is going to understand the bundle2 header. We move the function to a more suitable place before making any other changes.