TODO Technically the proper way to do this test is to use a
RevWalk on \"$id --not --all\" and test for an empty set. But
that is way slower than looking for a ref directly pointing
at the desired tip. We should always have a ref available.

TODO this is actually an error, the branch is gone but we
want to merge the issue. We can't safely do that if the
tip is not reachable.
