Fix the error message logged when omnibus failed to resolve refs to commits

Hi, 

Thanks for sharing this!

I doubt that omnibus emits an incorrect message when it fails to resolve refs(typically specified via `version` in `config/software/foo.rb`) to commit IDs.

I get "Failed to fetch git repository '#{@source[:git]}'", but what I think better is something like "Failed to find any commits for the ref '#{ref}'", since in this case omnibus did succeed to fetch.

This PR will fix the message while adding specs to GitFetcher to ensure that it work before and after the fix.

Regards,
Yusuke