untracked cache: save to an index extension

Helped-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/dir.h b/dir.h
index 95baf01..dc3ee0b 100644
--- a/dir.h
+++ b/dir.h
@@ -298,4 +298,5 @@
 			      has_trailing_dir);
 }
 
+void write_untracked_extension(struct strbuf *out, struct untracked_cache *untracked);
 #endif