Fix issue #1314

Don't change `dirty` or notify for a change when we set the same bit twice with `setbit`.

`setbit` should work like [`sadd`](https://github.com/antirez/redis/blob/73a809b1591378e1042a1028d0b8e10217e6e7c7/src/t_set.c#L250) which also doesn't mark anything dirty or send any notification when nothing changes.