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

https://bz.mercurial-scm.org/1089

  $ hg init
  $ mkdir a
  $ echo a > a/b
  $ hg ci -Am m
  adding a/b

  $ hg rm a
  removing a/b
  $ hg ci -m m a

  $ mkdir a b
  $ echo a > a/b
  $ hg ci -Am m
  adding a/b

  $ hg rm a
  removing a/b
  $ cd b

Relative delete:

  $ hg ci -m m ../a

  $ cd ..