Rename receive-objects command to receive-objects-v2.

Rename receive-objects command to receive-objects-v2.

...since it's incompatible with the old one.  That will make it die more
spectacularly when talking to an old-style server, rather than failing in
more confusing ways.

Theoretically we could do fancy things like making our server support both
variants of receive-objects, but hey, bup is a pre-release, it shouldn't be
acquiring backwards compatibility cruft *already* :)

Signed-off-by: Avery Pennarun <apenwarr@gmail.com>