Handle/understand symbolic/short-lived refnames

Symbolic refnames such as `HEAD`, `ORIG_HEAD`, `REBASE_HEAD` and such [undergo special handling by git](https://git-scm.com/docs/gitrevisions#Documentation/gitrevisions.txt-refnameegmasterheadsmasterrefsheadsmaster). Apply the documented refname handling to better show branch status in the default output of `git overview` .