commit-graph: use size_t for array allocation and indexing

commit-graph: use size_t for array allocation and indexing

Our packed_commit_list is an array of pointers to commit structs. We use
"int" for the allocation, which is 32-bit even on 64-bit platforms. This
isn't likely to overflow in practice (we're writing commit graphs, so
you'd need to actually have billions of unique commits in the
repository). But it's good practice to use size_t for allocations.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>