Fixes #102

Essentially, branched iteration order is arbitrary, we happen to choose a path (which is the path rendered by alembic history), but the "current" revision at each stop point is ambiguous. So instead, we need to keep track of the source and dest revisions at each stop point and supply that as the "current" revision.

This is only meaningful for the built-in "up_down_consistency" test, because it's the only one that upgrades each revision individually.