Fix newvers.sh to no longer print an outdated SVN rev

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 255337).

Also turn the format more into what git-describe produces, with a name
first, then the number of commits 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
FreeBSD 12.2-STABLE #0 stable/12-c243035-gd16dac42b641-dirty

MFC after: 3 weeks
Reviewed by: imp, glebius
Differential Revision: https://reviews.freebsd.org/D27751