Consolidate handling of SSH usernames
Summary:
Ref T4292. This consolidates code for figuring out which user we should connect to hosts with.
Also narrows a lock window.
Test Plan: Browsed Diffusion, pulled and pushed through an SSH proxy.
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T4292
Differential Revision: https://secure.phabricator.com/D15754