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
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