[repo] move repo stats to Service (closes #2951067)
We currently add a new method on repo that anyone can call. Call service allows
to reach the same result with:
* Restricted access to manager only (thanks to selection on services),
* Less unrelated code on the repo.
* No need to fetch a reference to a repo object from client side.
`req._cnx.repo` is not an API and config.repository() is on its way out.
The old way to access this information (repo.stats()) is deprecated.