newvers.sh: Avoid spurious -dirty in git revision

If git is installed and .git exists but git rev-parse failed to report a
hash we previously produced just "-dirty" as the git revision. Gate the
git commit count and -dirty check on the rev-parse passing.

Reviewed by: jlduran
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D57995