upgrade: make dirstate-v2 format variant compatible with share This only affects the dirstate and is safe to upgrade in the share. Differential Revision: https://phab.mercurial-scm.org/D12197

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# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL