If possible, use "differential.revision.edit" in "arc diff"

Summary:
Depends on D18741. Switches to "differential.revision.edit" if we get a transaction list back from parsing (added by D18740).

This will allow us to use new transactions, notably "Hold as Draft" for "--draft".

Test Plan: Ran arc diff, got a revision via differential.revision.edit.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18742