Builds of opam from git tags should not treat themselves as dev versions

[dra27]

Related to #4554 (comment), this has come up with the docker-base-images at ocaml/opam as well. If opam config report includes a git sha with the version of opam then that opam binary regards it itself as a dev version. This is problematic for hard upgrades, as these always prompt interactively on dev versions.

It's also philosophically problematic that a release tarball builds different from an otherwise identical checkout of the release tag in a git repository.

The solution: if git describe --exact reveals that this is a tag, treat it as a release (cf. the use of VERSION to do very similar, just by hand, in ocaml/ocaml)