Schedule repository updates from arc commit, not just arc land

Summary: Ref T5926. We only pass Phabricator an update hint from arc land, not from arc commit.

Test Plan:

• Ran arc land and arc commit with --trace, saw hints go over the wire.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5926

Differential Revision: https://secure.phabricator.com/D10324