largefiles: test interaction between largefiles and vanilla clients and servers This reintroduces tests for the case described by issue3066, among others: - vanilla clients and servers should still be able to talk to largefiles clients and servers about vanilla repos - vanilla clients should not attempt to clone largefiles repos over http and should report a useful error message (issue3066; this appears to work correctly) - vanilla clients should not attempt to clone largefiles repos over ssh, again with a useful error message - largefiles clients should not attempt to push largefile changes to vanilla servers