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
#require docutils gettext
$ $TESTDIR/check-gendoc it
checking for parse errors