dirstate: make dir collision logic faster
- shortcircuit decpath if we haven't built the _dirs map
- increment only for leafnodes of directory tree
(this should make construction more like O(nlog n) than O(n^2))
adding a
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
adding b
pushing to ../a
abort: could not lock repository ../a: Permission denied