Allow "Abandoned" revisions to be commandeered

Summary:
Ref T13216. See PHI985. You currently can't commandeer an abandoned revision, but this workflow is perfectly fine.

The caution here is just around weird use cases where, e.g., users want to reopen a revision to add a revert to it. These workflows tend to create problems so we try to guide users away from them.

Test Plan:

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13216

Differential Revision: https://secure.phabricator.com/D19835