Add git vcsversion method.
Add git vcsversion method.

Leaving in the branch_nick for now - I think we need to go through the
projects using this code and ensure that they aren't going to get screwed
before we remove it.

Change-Id: Ib91b5af050244f44cd811ca47cb6e2c53ef74ddb

openstack-attic/oslo.version,emonty/oslo.version