tree: convert read_tree_recursive to struct object_id

Convert the callback functions for read_tree_recursive to take a pointer
to struct object_id.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/merge-recursive.c b/merge-recursive.c
index 6ff971f..c886f2e 100644
--- a/merge-recursive.c
+++ b/merge-recursive.c
@@ -335,7 +335,7 @@ struct tree *write_tree_from_memory(struct merge_options *o)
 	return result;
 }
 
-static int save_files_dirs(const unsigned char *sha1,
+static int save_files_dirs(const struct object_id *oid,
 		struct strbuf *base, const char *path,
 		unsigned int mode, int stage, void *context)
 {