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

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# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL