Support fetching the memory usage of a delta index

Delta indices, at least on 64-bit platforms, tend to be larger than
the actual uncompressed data.  As such, keeping track of this storage
is important if you want to successfully limit the memory size of your
pack window.

Squirrel away the total allocation size inside the delta_index struct,
and add an accessor "sizeof_delta_index" to access it.

Signed-off-by: Brian Downing <bdowning@lavos.net>
Acked-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>