The deal with bundles is: they really are thin packs, with very little
sugar on top. So we really need a repository (or more appropriately, an
object database) to work with, when asked to verify a bundle.

Let's error out with a useful error message if `git bundle verify` is
called without such an object database to work with.

Reported by Konstantin Ryabitsev.
Signed-off-by: NJohannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>