Allow amending revisions without commandeering first
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](https://phabricator.wikimedia.org/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

r4nt/arcanist,codelabs-ch/arcanist,wikimedia/phabricator-arcanist,fivestars/arcanist,phacility/arcanist,haskell-infra/arcanist,codelabs-ch/arcanist,wikimedia/phabricator-arcanist,wikimedia/phabricator-arcanist,folsom-labs/arcanist,uber/arcanist,phacility/arcanist,Khan/arcanist,codelabs-ch/arcanist,Khan/arcanist,phacility/arcanist,Khan/arcanist,folsom-labs/arcanist,aik099/arcanist,Khan/arcanist,folsom-labs/arcanist,uber/arcanist,chundongwang/arcanist,uber/arcanist,aik099/arcanist,haskell-infra/arcanist,phacility/arcanist,fivestars/arcanist,haskell-infra/arcanist,fivestars/arcanist,chundongwang/arcanist,codelabs-ch/arcanist,codelabs-ch/arcanist,r4nt/arcanist,uber/arcanist,folsom-labs/arcanist,aik099/arcanist,haskell-infra/arcanist,r4nt/arcanist,aik099/arcanist,phacility/arcanist,chundongwang/arcanist,r4nt/arcanist,wikimedia/phabricator-arcanist,fivestars/arcanist,chundongwang/arcanist