When a repo is cloned via http and the remote repo has an out-dated
_darcs/packs then we get only the version that is in the packs and not
the current one. This is the case even though we fetch and unpack the
patches pack in parallel to getting them normally. The reason is that
the _darcs/hashed_inventory that we use to determine the patches to
download is part of the basic pack and thus out-dated, too. To fix this,
we need to download _darcs/hashed_inventory separately.