allow git-bundle to create bottomless bundle

allow git-bundle to create bottomless bundle

While "git bundle" was a useful way to sneakernet incremental
changes, we did not allow:

	$ git bundle create v2.6.20.bndl v2.6.20

to create a bundle that contains the whole history to a
well-known good revision.  Such a bundle can be mirrored
everywhere, and people can prime their repository with it to
reduce the load on the repository that serves near the tip of
the development.

Signed-off-by: Junio C Hamano <gitster@pobox.com>