dirstate-v2: fix infinite loop in pure packer Due to the naive approach to path relative-ness, some tree shapes like the one introduced in the associated test could result in the packer going into an endless loop which allocated new `Node` objects endlessly until the process was killed by Linux's OOM killer. Differential Revision: https://phab.mercurial-scm.org/D12170

  $ LANG=nonexistent LC_ALL=nonexistent LANGUAGE=nonexistent hg version -q
  Mercurial Distributed SCM (version *) (glob)