mark unused read_tree_recursive() callback parameters

We pass a callback to read_tree_recursive(), but not every callback
needs every parameter. Let's mark the unused ones to satisfy
-Wunused-parameter.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

merge-recursive.c

@@ -456,7 +456,7 @@ static void unpack_trees_finish(struct merge_options *opt)
 	clear_unpack_trees_porcelain(&opt->priv->unpack_opts);
 }
 
-static int save_files_dirs(const struct object_id *oid,
+static int save_files_dirs(const struct object_id *UNUSED(oid),
 			   struct strbuf *base, const char *path,
 			   unsigned int mode, void *context)
 {