unpack_delta_entry(): reduce memory footprint.

unpack_delta_entry(): reduce memory footprint.

Currently we unpack the delta data from the pack and then unpack
the base object to apply that delta data to it.  When getting an
object that is deeply deltified, we can reduce memory footprint
by unpacking the base object first and then unpacking the delta
data, because we will need to keep at most one delta data in
memory that way.

Signed-off-by: Junio C Hamano <junkio@cox.net>