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-files.c

@@ -47,5 +47,9 @@ int cmd_diff_files(int argc, const char **argv, const char *prefix)
 	if (rev.pending.nr ||
 	    rev.min_age != -1 || rev.max_age != -1)
 		usage(diff_files_usage);
+	if (read_cache() < 0) {
+		perror("read_cache");
+		return -1;
+	}
 	return run_diff_files(&rev, silent);
 }