dirstate._dirs: fix refcounting broken by 7dfac37cfabf
reported by Patrick Waugh
% try to commit on top of a patch
abort: cannot commit over an applied mq patch
% qpop/qrefresh on the wrong revision
abort: working directory revision is not qtip
using patch queue:
abort: popping would remove a revision not managed by this patch queue
abort: working directory revision is not qtip
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
% qpop
abort: popping would remove a revision not managed by this patch queue
% qrefresh
abort: cannot refresh a revision with children
% tip:
3 append quux