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