ipfs dag stat improperly return size of dags that include the same blocks multiple time as different codecs

### Checklist

- [X] This is a bug report, not a question. Ask questions on [discuss.ipfs.io](https://discuss.ipfs.io).
- [X] I have searched on the [issue tracker](https://github.com/ipfs/go-ipfs/issues?q=is%3Aissue) for my bug.
- [X] I am running the latest [go-ipfs version](https://dist.ipfs.io/#go-ipfs) or have an issue updating.

### Installation method

built from source

### Version

```Text
v0.12.0
```

### Config

```json
N/A
```

### Description

A dag like this:
```
Qmfoo -> bafkreibar
Qmfoo -> bafybeibar
bafybeibar -> bafkreibaz
```
Will count `bafkreibar` and `bafybeibar` twice even tho since v0.12.0 and multihash datastore keying it is actually only stored once.