Depends on D18740. Prepares arc to receive a --draft flag by letting us switch to "differential.revision.edit" instead of "differential.createrevision".

To "differential.revision.edit", we need a transaction list, but we can't automatically construct this list from a field map. Return the transaction list alongside the field map.

The next change uses this list (if available) to switch us to the modern API method.

Ran arc diff on the experiemntal branch with followup changes, got a new revision.