Allows branches to appear in diffusion with out triggering closing things from the commit message

Summary: Implemented it how it was suggested in ticket comments

Test Plan: create a revision in a branch, push that branch up, verify it's visible in diffusion and also that revision is not closed, then merge and push to master, verify that revision closed

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin, 20after4

Maniphest Tasks: T1210

Differential Revision: https://secure.phabricator.com/D2706