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.