diff: make -O and --output work in subdirectory

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/diff.h b/diff.h
index f61ee54..76b5536 100644
--- a/diff.h
+++ b/diff.h
@@ -268,7 +268,7 @@
 extern int git_diff_basic_config(const char *var, const char *value, void *cb);
 extern int git_diff_ui_config(const char *var, const char *value, void *cb);
 extern void diff_setup(struct diff_options *);
-extern int diff_opt_parse(struct diff_options *, const char **, int);
+extern int diff_opt_parse(struct diff_options *, const char **, int, const char *);
 extern void diff_setup_done(struct diff_options *);
 
 #define DIFF_DETECT_RENAME	1