R1:4faa4b451f41

When viewing a branch, preview differences from master

Summary: Ref T929. When viewing a branch, show a few recent differences from the default branch (usually, "master").

Test Plan: {F2079220}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T929

Differential Revision: https://secure.phabricator.com/D16991