Derive ref-spec from local HEAD

[alilleybrinker]

Currently, if you give Hipcheck a local repo to analyze that's checked out to an old ref, we'll produce an unclear error message. If you pass the --ref flag to the current HEAD, we'll use it. We should produce a clearer error message when the local repo is out-of-date, explaining that if you explicitly want to analyze this older state to pass the --ref flag explicitly.

Edit: Based on feedback, we are updating this issue to instead not require a --ref flag under these circumstances. Instead, Hipcheck should determine the correct ref-spec from the current HEAD, since it should be unambiguous.