reftable/stack: add accessor for the hash ID

Add an accessor function that allows callers to access the hash ID of a
reftable stack. This function will be used in a subsequent commit.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

reftable/stack.c

@@ -1791,3 +1791,8 @@ done:
 	reftable_addition_destroy(add);
 	return err;
 }
+
+enum reftable_hash reftable_stack_hash_id(struct reftable_stack *st)
+{
+	return reftable_merged_table_hash_id(st->merged);
+}