After landing to a repository, hint that it should be updated

Summary: Fixes T4605. Smart waits (see D8782) are presumably good on the balance, but may cause some delays when changes are made to rarely updated repositories. To help mitigate this, have arc land hint that a repository has changed.

Test Plan: Will run arc land.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4605

Differential Revision: https://secure.phabricator.com/D8783