Handle empty git repos

[sp3nx0r]

Addresses Issue #19

Two changes:

1. Add exception catching for _git() for two scenarios: A) initial add (fails on getting HEAD) and B) scan with empty commit hashes
2. Add code to handle scanning an empty repo and update said repo if it does flip to non-empty in scan.py. This also required a change in _clone_and_save_repo() to allow an update to existing metadata.

Unintended side effect was having to change the test case for existing metadata. I flipped it to testing for allowing update because my mocking blows and I couldn't figure out how to easily add a repo.last_commit_hash so it wouldn't take the empty repo path.

[KevinHock]

Looks really great to me, thank you for making this!

Asked for one clarification, but that's all.