`ipfs dag stat` should return the compression ratio

### Checklist

- [X] My issue is specific & actionable.
- [X] I am not suggesting a protocol enhancement.
- [X] I have searched on the [issue tracker](https://github.com/ipfs/go-ipfs/issues?q=is%3Aissue) for my issue.

### Description

I would find neat if `ipfs dag stat` also displayed a float `CumulatedSize (red from the block metadata when present) / Size (true size of IPFS blocks)`.
This would go below 1 for files that have a slight overhead due to roots taking up place.
And above 1 for things that reuse some blocks.

That is really easy to do as `ipfs object stat` already display the `CumulatedSize`.

The only thing I'm unsure if we want that, is the name, "compression ratio" sounds like we try to save space, while we don't it's mainly luck.