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
python -c 'print "confuse str.splitlines\nembedded\rnewline"' > a
hg ci -Ama -d '1 0'
echo clean diff >> a
hg ci -mb -d '2 0'
hg diff -r0 -r1