connect: move no_fork fallback to git_tcp_connect

connect: move no_fork fallback to git_tcp_connect

git_connect has the structure

	struct child_process *conn = &no_fork;

	...
	switch (protocol) {
	case PROTO_GIT:
		if (git_use_proxy(hostandport))
			conn = git_proxy_connect(fd, hostandport);
		else
			git_tcp_connect(fd, hostandport, flags);
		...
		break;
	case PROTO_SSH:
		conn = xmalloc(sizeof(*conn));
		child_process_init(conn);
		argv_array_push(&conn->args, ssh);
		...
		break;
	...
	return conn;

In all cases except the git_tcp_connect case, conn is explicitly
assigned a value. Make the code clearer by explicitly assigning
'conn = &no_fork' in the tcp case and eliminating the default so the
compiler can ensure conn is always correctly assigned.

Noticed-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>