indicate when a build is relatively out of date

[avsm]

The index doesn't currently store the opam-repository metadata, but it does have the revision of opam-repo that a given run was built against. Some runs (like ppc64le) naturally lag behind the faster amd64 builds, so different results come from different revisions of opam-repo.

This is problematic when reporting results, since we want to indicate to the user that a given branch may be lagging behind. We could extend the type pkgs (currently just a pkg list) to also include more information about the opam-repository revision hashes. Then the client could determine which the latest revision is, and report the mismatch in opam-ci logs without having to have a full checkout of opam-repository.

[avsm]

requested by @samoht