Merge branch 'js/checkout-detach-count' into maint

When checking out another commit from an already detached state, we used
to report all commits that are not reachable from any of the refs as
lossage, but some of them might be reachable from the new HEAD, and there
is no need to warn about them.

By Johannes Sixt
* js/checkout-detach-count:
  checkout (detached): truncate list of orphaned commits at the new HEAD
  t2020-checkout-detach: check for the number of orphaned commits