Make "arc land" support landing someone else's code that got there via arc patch

Summary: Ref T4670.

Test Plan: arc patch D8685; arc land --hold; verified i got a nice message asking me to be sure i wanted to land epriestley's code

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T4670

Differential Revision: https://secure.phabricator.com/D8709
Merge branch 'master' of http://github.com/facebook/arcanist