make "Land to..." button call diffusion.looksoon

Summary:
add looksoon call after every attempt at landing.

This includes failed attempts, to elevate "not a fast-forward" issues, although there are probably smarter things to be done about that.

Test Plan: Land, look at logs.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9518