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