diff-lib: convert do_diff_cache to struct object_id

This is needed to convert parse_tree_indirect.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/am.c b/builtin/am.c
index 642d704..200d9db 100644
--- a/builtin/am.c
+++ b/builtin/am.c
@@ -1145,7 +1145,7 @@ static int index_has_changes(struct strbuf *sb)
 		DIFF_OPT_SET(&opt, EXIT_WITH_STATUS);
 		if (!sb)
 			DIFF_OPT_SET(&opt, QUICK);
-		do_diff_cache(head.hash, &opt);
+		do_diff_cache(&head, &opt);
 		diffcore_std(&opt);
 		for (i = 0; sb && i < diff_queued_diff.nr; i++) {
 			if (i)