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.