Currently the git-plugin only supports the SCM API for creating GitSCMFileSystem's if the ref specified is a branch and begins with ref/heads.
It would be beneficial if tags could also be supported.
I am opening a PR that addresses this and look forward to the discussion!
Expected to be available in git plugin 4.2.0 approxiimately end of Feb 2020
Released in git plugin 4.2.0 March 1, 2020