Specify an ssh port for Diffusion when running against the grain

Summary: We run `git` on a different port than 22, so would like to reflect this change in the UI.

Test Plan: Set diffusion.ssh-port in settings, then make sure it's reflected on the Diffusion repository Clone URI.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran, dctrwatson

Differential Revision: https://secure.phabricator.com/D7755