Fix for reporting dirty when the commit count is zero

[jlstevens]

Also updates one outdated ref. Addresses #43

[jlstevens]

@ceball Looks like the tests will all go green now.

[ceball]

I can't review the actual change to autover (I looked but am slightly confused about Version.commit_count, Version. _commit_count, Version.fetch()._commit_count). But the change seems to work so I'd be happy to see it merged.

[jlstevens]

Version.commit_count is the public property, Version._commit_count is the internal state that caches the value after calling git describe with fetch() and Version.fetch()._commit_count is the internal value after a fetch.

It is just a simple way to avoid calling git describe excessively which is expensive to do. Using the properties is always safe.