More information on opam repo list or add opam repo info

[dbuenzli]

Currently it's quite hard or impossible to know at which commit an opam repository is. I think opam repo list:

> opam repo list     
[NOTE] These are the repositories in use by the current switch. Use '--all' to see all configured repositories.

<><> Repository configuration for switch 4.13.0 <><><><><><><><><><><><><><><><>
 1 beta     git+https://github.com/ocaml/ocaml-beta-repository.git
 2 ocamlnat git+https://github.com/dbuenzli/opam-repo-ocamlnat.git
 3 default  https://opam.ocaml.org/

could add a column after the name with the short hash at which the repo is. For now the only place I could find the information is in opam config report and only for the default repository:

> opam config report | grep repositories 
# repositories         1 (http), 1 (local), 2 (version-controlled) (default repo at b3f8647a)

Alternatively if there's more useful information about a repo one might want to know maybe an opam repo info command could be warranted.

Thanks.

/cc @kit-ty-kate