(stable) Fix a MarkerRef call to get the active bookmark in Mercurial
(stable) Fix a MarkerRef call to get the active bookmark in Mercurial

Summary: See PHI1805. This call is constructed improperly and can lead to a fatal in `arc patch` under Mercurial.

Test Plan: In Mercurial, ran a valid `arc patch` operation.

Differential Revision: https://secure.phabricator.com/D21391

codelabs-ch/arcanist,codelabs-ch/arcanist,codelabs-ch/arcanist,codelabs-ch/arcanist,codelabs-ch/arcanist