cat-file: add %(objectsize:disk) format atom

cat-file: add %(objectsize:disk) format atom

This atom is just like %(objectsize), except that it shows
the on-disk size of the object rather than the object's true
size. In other words, it makes the "disk_size" query of
sha1_object_info_extended available via the command-line.

This can be used for rough attribution of disk usage to
particular refs, though see the caveats in the
documentation.

This patch does not include any tests, as the exact numbers
returned are volatile and subject to zlib and packing
decisions. We cannot even reliably guarantee that the
on-disk size is smaller than the object content (though in
general this should be the case for non-trivial objects).

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