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.