run_diff_{files,index}(): update calling convention.

They used to open and read index themselves, but they now expect
their callers to do so.

Signed-off-by: Junio C Hamano <junkio@cox.net>

builtin-diff-index.c

@@ -38,5 +38,9 @@ int cmd_diff_index(int argc, const char **argv, const char *prefix)
 	if (rev.pending.nr != 1 ||
 	    rev.max_count != -1 || rev.min_age != -1 || rev.max_age != -1)
 		usage(diff_cache_usage);
+	if (read_cache() < 0) {
+		perror("read_cache");
+		return -1;
+	}
 	return run_diff_index(&rev, cached);
 }