dirstate: move verification code within the dirstate itself This move implementation details further down the stack and make it the verification code easier to discover. Differential Revision: https://phab.mercurial-scm.org/D11526

dirstate-entry: use `?` for the state of entry without any tracking This is what the dirstate use at a higher level. Differential Revision: https://phab.mercurial-scm.org/D11525

dirstate-item: introduce a `any_tracked` property This property is True is the file is tracked anywhere, either the working copy, or any of the parent. Differential Revision: https://phab.mercurial-scm.org/D11524