"git checkout/switch --detach foo", after switching to the detached
HEAD state, gave the tracking information for the 'foo' branch,
which was pointless.
Tested-by: M Hickford <mirth.hickford@gmail.com>
cf. <CAGJzqsmE9FDEBn=u3ge4LA3ha4fDbm4OWiuUbMaztwjELBd7ug@mail.gmail.com>
* jc/checkout-detach-wo-tracking-report:
checkout: omit "tracking" information on a detached HEAD