dir.c: add free_excludes()

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/unpack-trees.c b/unpack-trees.c
index 803445a..4d92412 100644
--- a/unpack-trees.c
+++ b/unpack-trees.c
@@ -945,11 +945,7 @@
 		*o->dst_index = o->result;
 
 done:
-	for (i = 0;i < el.nr;i++)
-		free(el.excludes[i]);
-	if (el.excludes)
-		free(el.excludes);
-
+	free_excludes(&el);
 	return ret;
 
 return_failed: