Octocat for 'Land to GitHub'
Summary: Adds an action icon for GitHub.
Test Plan: I couldnt see how to update that icon in Differential. But. Photoshop.
Reviewers: epriestley
Reviewed By: epriestley
CC: Korvin, epriestley, aran
Differential Revision: https://secure.phabricator.com/D7635