newvers.sh: Avoid spurious -dirty in git revision

[emaste]

Summary

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.

[gordon]

jhb might have some views here too.

[jlduran]

This is fine by me.
So, if git-rev-parse(1) fails, we just silently ignore it? Maybe erroring in the else branch is too aggressive.