Update PhabricatorRepositoryManagementLookupUsersWorkflow to use ConduitCall

Summary:
Ref T2783.

This updates PhabricatorRepositoryManagementLookupUsersWorkflow to use ConduitCall to retrieve information about the commit.

Test Plan:
Ran bin/repository lookup-users rTESTe9683b64d3283f0b2d355fdbf231bc918b5ac0ab --trace and saw the information returned (by making a request to diffusion.querycommits as the omnipotent user, signed with the device key).

Mucked with cluster.addresses and saw requests rejected.

Reviewers: hach-que, btrahan

Reviewed By: btrahan

Subscribers: Krenair, epriestley, Korvin

Maniphest Tasks: T2783

Differential Revision: https://secure.phabricator.com/D10403