Retrieve unannotated Git tags too

[wraithgar]

Unannotated Git tags should be considered alongside annotated tags when
retrieving a version from Git.

Compared to annotated tags, unannotated tags do not contain a distinct
commit message or metadata, but neither are needed to retrieve version name.

See: https://git-scm.com/docs/git-describe#Documentation/git-describe.txt---tags

References

Cherry-pick from #5

[wraithgar]

@isaacs and @nlf both reviewed this out of band.