Summary:
Allow querying and modifying flags from arcanist. Currently
supports only printing and deleting flags for Differential revisions,
but it should be straightforward to add more capabilities (given Conduit
support).
Test Plan:
Run arc flag, passing it various revisions. Flags are
modified appropriately.
Reviewers: epriestley
Reviewed By: epriestley
CC: aran, Korvin
Maniphest Tasks: T1556
Differential Revision: https://secure.phabricator.com/D3133
