Allow amending revisions without commandeering first

Summary:
It is common practice in Wikimedia's projects to amend a contributor's
change without taking over authorship of the change. We found that
the only enforcement of commandeering before amending is in arcanist,
not validated server-side. While it would be fairly straightforward to
maintain this as a patch to arcanist, I thought I would see if upstream
is willing to support making this optional.

With this change, amending without commandeering is enabled by a flag in
.arcconfig and it defaults to the old behavior.

For background see wmf T121751

Test Plan:

• ran arc patch D146 to locally apply a revision that I did not author,
• made a trivial change and amended the commit.
• ran arc diff --update D146 HEAD^ to send the update to differential
• Saw that https://phabricator.wikimedia.org/D146 updated as it should.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: greggrossmeier, aklapper, Luke081515.2, Korvin, dereckson

Maniphest Tasks: T10584

Differential Revision: https://secure.phabricator.com/D15468