It would be nice to have `git annex unused` report the total size of unused files.

I tried:

- ❌ `git annex unused`: doesn't show the size
- ❌ `git annex info`: doesn't list unused size
- ❌ `git annex info --unused`: doesn't work
- ❌ `git annex list --unused`: doesn't work
- ❌ `git annex find --unused`: doesn't work

Maybe I'm missing something, but it feels like having `git annex unused` and `git annex info` directly report it would be nice.

Cheers,
Yann
