dirstatemap: add a common `_refresh_entry` method for dirstatemap
This method is called once a DirstateItem have been modified to apply the
change on the dirstatemap if necessary.
Each variant have a different implementation (which is … the point).
We use `addfile` for the rustmap and not `set_dirstate_item` because we need to
keep the internal counter up to date and `set_dirstate_item` does not do it.
Differential Revision: https://phab.mercurial-scm.org/D11571
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