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.
all()
draft()
::tip
draft() and ::tip
0::tip
roots(0::tip)
author(lmoscovicz)
author(mpm)
author(lmoscovicz) or author(mpm)
tip:0
max(tip:0)
min(0:tip)
0::
min(0::)
roots((tip~100::) - (tip~100::tip))
::p1(p1(tip))::