We need this to populate the cache_revision_origin table of the content provenance information database.

Of course, a first (easy) step is doing the addition for the first visit.

Some interesting cases arise when we consider what to do between two subsequent visits:

• A new branch appears. Do we want to register all the history as belonging to the new branch, or just the new revisions?
• A branch disappears. Do we record that some merged revisions appeared in the destination branch?
• Or do we keep things simple and only ever record revisions that we haven't recorded before in the history of the given origin?

The choice boils down to either:

• computing the differences branch per branch
• computing the differences globally