index-pack: Refactor base arguments of resolve_delta into a struct

index-pack: Refactor base arguments of resolve_delta into a struct

We need to discard base objects which are not recently used if our
memory gets low, such as when we are unpacking a long delta chain
of a very large object.

To support tracking the available base objects we combine the
pointer and size into a struct.  Future changes would allow the
data pointer to be free'd and marked NULL if memory gets low.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>