We have stopped using SVN, so the notes containing the old SVN revisions
are no longer populated, so fall back to purely counting the number of
commits (currently at about 255126).
Also turn the format more into what git-describe produces, with a name
first, then potentially a number of commits on top of that and the hash
last. Note that as we don't tag anything on main, git describe will
never produce something useful there and finds the newest vendor tag
that was merged in instead.
Sample output: FreeBSD 13.0-CURRENT #6 main-c255126-gb81783dc98e6-dirty