Return transactions from "differential.parsecommitmessage"

Summary:
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.

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

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18741