index-pack: Chain the struct base_data on the stack for traversal

index-pack: Chain the struct base_data on the stack for traversal

We need to release earlier inflated base objects when memory gets
low, which means we need to be able to walk up or down the stack
to locate the objects we want to release, and free their data.

The new link/unlink routines allow inserting and removing the struct
base_data during recursion inside resolve_delta, and the global
base_cache gives us the head of the chain (bottom of the stack)
so we can traverse it.

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