Converts a tagged migration list to the one object the upload API takes.
Returns undefined when there is nothing to apply: an empty list, or a Worker that already carries
the last tag. A tag the list does not hold (it may have been pruned) applies every step.
Converts a tagged migration list to the one object the upload API takes.
Returns undefined when there is nothing to apply: an empty list, or a Worker that already carries the last tag. A tag the list does not hold (it may have been pruned) applies every step.