dirstate-item: add dedicated "legacy" constructor for `addfile` case This way the internal details of how a DirstateItem is encoded is encapsulated within the DirstateItem. This will finally give use some latitude to change the data we store in a DirstateItem. The addfile logic will likely be rewritten eventually and these dedicated constructor can be removed at that time. In the mean-time this should help with hiding internal details of DirstateItem and to migrate it to new internal storage and logic. Differential Revision: https://phab.mercurial-scm.org/D11330