test-issue660: add dirstate-v2 variant
It's basically a dirstate test, so it makes sense to test out the new version.
Differential Revision: https://phab.mercurial-scm.org/D12445
## Avoid interference from actual test env:unset HGTEST_JOBSunset HGTEST_TIMEOUTunset HGTEST_PORTunset HGTEST_SHELL