Test dirstate._dirs refcounting:

  $ hg init t

  $ cd t

  $ mkdir -p a/b/c/d

  $ touch a/b/c/d/x

  $ touch a/b/c/d/y

  $ touch a/b/c/d/z

  $ hg ci -Am m

  adding a/b/c/d/x

  adding a/b/c/d/y

  adding a/b/c/d/z

  $ hg mv a z

  moving a/b/c/d/x to z/b/c/d/x

  moving a/b/c/d/y to z/b/c/d/y

  moving a/b/c/d/z to z/b/c/d/z