If commit flag is not set use HEAD instead

[lwsanty]

Use either

• go-git functionality
• git rev-parse HEAD

[creachadair]

I have a mild preference for requiring the commit flag to be set. We could allow it to be set explicitly to a symbolic reference (just use git rev-parse on whatever the user enters, and log the result—but having that be an explicit choice means a typescript of a testing session will record intent.