bundle: allow bundle command to use changegroup3 in tests
Since bundle2 writes changegroup version, we can just reuse the bundle2
format for changegroup3.
This won't enable the bundle command to write changegroup3 in the wild,
since exchange.parsebundlespec only returns changegroup2. It unlocks tests
to override exchange.parsebundlespec and get "hg bundle" write changegroup3.
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.