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.
Reviewed By: btrahan
Maniphest Tasks: T4605
Differential Revision: https://secure.phabricator.com/D8783