dirstate: introduce an internal `_add` method We want to split current user of `dirstate.add` between `hg add`-like cases and update of the dirstate coming from update/merge. To do this we will introduce new API. The first step is to introduces an internal function that these new API migh use (or not use) to distinct between the migrated users and the others. Differential Revision: https://phab.mercurial-scm.org/D11010