DRF sorter may omit clients in sorting after removing an inactive leaf node.

The sorter assumes inactive leaf nodes are placed in the tail in the children list of a node.
However, when collapsing a parent node with a single "." virtual child node, its position may fail to be updated due to a bug in `Sorter::remove()`:

CHECK(child->isLeaf());
....
current->kind = child->kind;
...
if (current->kind == Node::INTERNAL) {
}

This bug would manifest, if
(1) we have a/b and a/.
(2) deactivate(a), i.e. a/. becomes inactive_leaf
(3) remove(a/b)
When these happens, a/. will collapse to `a` as an inactive_leaf, due to the bug above, however, it will not be placed at the end, resulting in all the clients after `a` not included in the sort().

Luckily, this should never happen in practice, because only frameworks will get deactivated, and frameworks don’t have sub clients.