Pinning does not handle repositories without any tags

[euanh]

On a repository with no tags, git describe HEAD fails:

$ git describe HEAD
fatal: No names found, cannot describe anything.

The --all and --long options return something more useful:

$ git describe --all --long HEAD
heads/master-0-gbc76db9

However to be useful as a version number for RPM upgrades, we need some sort of increasing number, such as the number of commits since the root commit. We might have to tag the root (temporarily) as 0.0.0 to get that.