struct ref_transaction: add a place for backends to store data

`packed_ref_store` is going to want to store some transaction-wide
data, so make a place for it.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs/refs-internal.h

@@ -242,6 +242,7 @@ struct ref_transaction {
 	size_t alloc;
 	size_t nr;
 	enum ref_transaction_state state;
+	void *backend_data;
 };
 
 /*