Jeff King authored Dec 21, 2013 and Junio C Hamano committed Dec 26, 2013

It can be useful for debugging or analysis to see which
objects are stored as delta bases on top of others. This
information is available by running `git verify-pack`, but
that is extremely expensive (and is harder than necessary to
parse).

Instead, let's make it available as a cat-file query format,
which makes it fast and simple to get the bases for a subset
of the objects.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>