dirstate-entry: add a `added` property
Lets use more semantic property instead of the `state` implementation details.
Differential Revision: https://phab.mercurial-scm.org/D10968
dirstate: use a `added` parameter to _addpath
My next target are the explicit `state` byte, so let start with abstracting
their initial storage.
Differential Revision: https://phab.mercurial-scm.org/D10967