commit-graph: not compatible with uninitialized repo

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

commit-graph.c

@@ -62,6 +62,9 @@ extern int read_replace_refs;
 
 static int commit_graph_compatible(struct repository *r)
 {
+	if (!r->gitdir)
+		return 0;
+
 	if (read_replace_refs) {
 		prepare_replace_object(r);
 		if (hashmap_get_size(&r->objects->replace_map->map))