Support "-" as a target for gg update

[zombiezen]

git checkout allows the user to switch back to the previous ref using a shorthand of git checkout - (docs). It can be implemented by rewriting the revision to @{-1}.

[zombiezen]

One wrinkle here: the fast-forwarding behavior of gg update causes the reflog to have a detached head for an entry in the reflog. Perhaps we need to find a way to remove the reflog entry?