Basically, there is no automated way to do this. Git revisions are not guaranteed to increment, so you'd end up with the SHA1 identifier of the commit rather than a number. Further, unlike in Subversion, your git checkout is identical to the content in the repo. There is no munging of the content on checkout like there is with Subversion, which is what allows keyword expansion.
I suggest disabling the policy. I use Perl::Version's perl-reversion script to keep track of my actual released versions. It's not as exact as a git identifier, but it's good.