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

"$TESTDIR/hghave" symlink || exit 80

hg init a
ln -s a link
cd a
echo foo > foo
hg status
hg status ../link