R1:d9296638cdd4

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…