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))

#!/bin/sh

hg init
touch a
hg add a
hg ci -m "a" -d "1000000 0"

echo 123 > b
hg add b
hg diff --nodates

hg diff --nodates -r tip

echo foo > a
hg diff --nodates

hg diff -r ""
hg diff -r tip -r ""

true