cat-file: provide %(deltabase) batch format

cat-file: provide %(deltabase) batch format

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>