subrepo: add tests for hg rogue ssh urls (SEC) 'ssh://' has an exploit that will pass the url blindly to the ssh command, allowing a malicious person to have a subrepo with '-oProxyCommand' which could run arbitrary code on a user's machine. In addition, at least on Windows, a pipe '|' is able to execute arbitrary commands. When this happens, let's throw a big abort into the user's face so that they can inspect what's going on.