dirstate: deprecate `__getitem__` access
If we want to drop `state` usage, we need to deprecate this.
Differential Revision: https://phab.mercurial-scm.org/D11544
## Avoid interference from actual test env:unset HGTEST_JOBSunset HGTEST_TIMEOUTunset HGTEST_PORTunset HGTEST_SHELL