Fix bug where rose-stem cannot ascertain name of a suite when cwd is not a working copy

If the current working directory where "rose stem" is invoked is not a working copy, the command will currently fail as it attempts to work out a sensible suite name from the base of the current working copy. 

This generally only occurs when running rose stem with both the --source and --config options, but that is occasionally necessary. This change alters the command so if it is not possible to ascertain a sensible basename of the working copy, it takes the directory specified by --config and uses the basename of that (replacing "." with the actual name), or failing that the basename of the directory in which the user ran the command.