commit-graph: read only from specific pack-indexes

Teach git-commit-graph to inspect the objects only in a certain list
of pack-indexes within the given pack directory. This allows updating
the commit graph iteratively.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/commit-graph.h b/commit-graph.h
index 73b28be..f065f08 100644
--- a/commit-graph.h
+++ b/commit-graph.h
@@ -36,6 +36,8 @@ struct commit_graph {
 
 struct commit_graph *load_commit_graph_one(const char *graph_file);
 
-void write_commit_graph(const char *obj_dir);
+void write_commit_graph(const char *obj_dir,
+			const char **pack_indexes,
+			int nr_packs);
 
 #endif