bookmark: prevent crashing when a successor is unknown locally (issue3680) The `%ln` revset substitution does not accept unknown node. We prune unknown node from potential successors before computing descendants. This have no impact on the result of this function. - Descendants of unknown changeset as unknown, - all successors of unknown changesets are already return by the call who returned those same unknown changesets, - unknown changesets are never a valid destination for a bookmark.