commit-graph: implement write_commit_graph()

Teach Git to write a commit graph file by checking all packed objects
to see if they are commits, then store the file in the given object
directory.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit-graph.h

@@ -0,0 +1,6 @@
+#ifndef COMMIT_GRAPH_H
+#define COMMIT_GRAPH_H
+
+void write_commit_graph(const char *obj_dir);
+
+#endif