Fix a "user@domain:path" protocol handling bug

Summary:
In D3063, we stopped converting "user@domain:path" git-style URIs, but this broke the SSH-detection code and I missed that in my test plan because my test case uses natural SSH keys so the omission of SSH flags didn't cause failures.

This code is a bit of a mess anyway. Consolidate and refactor it to be a bit simpler, and add test coverage.

Test Plan: Ran unit tests. Ran "test_connection.php" in SSH and non-SSH modes, verified SSH modes generated appropriate ssh-agent commands around the git remote commands.

Reviewers: vrana, btrahan, tberman

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1529

Differential Revision: https://secure.phabricator.com/D3093