dir: add remove_untracked_cache()

Factor out code into remove_untracked_cache(), which will be used
in a later commit.

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 cfd3636..a3dacdb 100644
--- a/dir.h
+++ b/dir.h
@@ -309,4 +309,5 @@
 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);
+void remove_untracked_cache(struct index_state *istate);
 #endif