unpack_entry(): convert to new revindex API

unpack_entry(): convert to new revindex API

Remove direct manipulation of the 'struct revindex_entry' type as well
as calls to the deprecated API in 'packfile.c:unpack_entry()'. Usual
clean-up is performed (replacing '->nr' with calls to
'pack_pos_to_index()' and so on).

Add an additional check to make sure that 'obj_offset()' points at a
valid object. In the case this check is violated, we cannot call
'mark_bad_packed_object()' because we don't know the OID. At the top of
the call stack is do_oid_object_info_extended() (via
packed_object_info()), which does mark the object.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>