Merge branch 'hn/checkout-track-fetch' into seen

Merge branch 'hn/checkout-track-fetch' into seen

"git checkout --track=..." learned to optionally fetch the branch
from the remote the new branch will work with.

* hn/checkout-track-fetch:
  checkout: extend --track with a "fetch" mode to refresh start-point
  branch: expose helpers for finding the remote owning a tracking ref