dir: add {new,add}_untracked_cache()

Factor out code into new_untracked_cache() and
add_untracked_cache(), which will be used
in later commits.

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/dir.h b/dir.h
index 7b5855d..cfd3636 100644
--- a/dir.h
+++ b/dir.h
@@ -308,4 +308,5 @@
 struct untracked_cache *read_untracked_extension(const void *data, unsigned long sz);
 void write_untracked_extension(struct strbuf *out, struct untracked_cache *untracked);
 void add_untracked_ident(struct untracked_cache *);
+void add_untracked_cache(struct index_state *istate);
 #endif