commit-graph: prepare for commit-graph chains

To prepare for a chain of commit-graph files, augment the
commit_graph struct to point to a base commit_graph. As we load
commits from the graph, we may actually want to read from a base
file according to the graph position.

The "graph position" of a commit is given by concatenating the
lexicographic commit orders from each of the commit-graph files in
the chain. This means that we must distinguish two values:

 * lexicographic index : the position within the lexicographic
   order in a single commit-graph file.

 * graph position: the position within the concatenated order
   of multiple commit-graph files

Given the lexicographic index of a commit in a graph, we can
compute the graph position by adding the number of commits in
the lower-level graphs. To find the lexicographic index of
a commit, we subtract the number of commits in lower-level graphs.

While here, change insert_parent_or_die() to take a uint32_t
position, as that is the type used by its only caller and that
makes more sense with the limits in the commit-graph format.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit-graph.h

@@ -48,6 +48,9 @@ struct commit_graph {
 	uint32_t num_commits;
 	struct object_id oid;
 
+	uint32_t num_commits_in_base;
+	struct commit_graph *base_graph;
+
 	const uint32_t *chunk_oid_fanout;
 	const unsigned char *chunk_oid_lookup;
 	const unsigned char *chunk_commit_data;