connect: factor out "looks like command line option" check

We reject hostnames that start with a dash because they may
be confused for command-line options. Let's factor out that
notion into a helper function, as we'll use it in more
places. And while it's simple now, it's not clear if some
systems might need more complex logic to handle all cases.

Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

connect.c

@@ -754,7 +754,7 @@ struct child_process *git_connect(int fd[2], const char *url,
 				return NULL;
 			}
 
-			if (ssh_host[0] == '-')
+			if (looks_like_command_line_option(ssh_host))
 				die("strange hostname '%s' blocked", ssh_host);
 
 			ssh = getenv("GIT_SSH_COMMAND");