* coverage/misc.py (ensure_dir): Pass exist_ok to os.makedirs, ensuring that if
two concurrent instances of coverage.py entering this function at the same time
won't fail with FileExistsError.

Sample backtrace:

    Traceback (most recent call last):
      File "/usr/lib/python3.8/runpy.py", line 194, in _run_module_as_main
        return _run_code(code, main_globals, None,
      File "/usr/lib/python3.8/runpy.py", line 87, in _run_code
        exec(code, run_globals)
      File "/home/user/.local/lib/python3.8/site-packages/coverage/__main__.py", line 8, in <module>
        sys.exit(main())
      File "/home/user/.local/lib/python3.8/site-packages/coverage/cmdline.py", line 871, in main
        status = CoverageScript().command_line(argv)
      File "/home/user/.local/lib/python3.8/site-packages/coverage/cmdline.py", line 588, in command_line
        return self.do_run(options, args)
      File "/home/user/.local/lib/python3.8/site-packages/coverage/cmdline.py", line 743, in do_run
        self.coverage.start()
      File "/home/user/.local/lib/python3.8/site-packages/coverage/control.py", line 535, in start
        self._init_for_start()
      File "/home/user/.local/lib/python3.8/site-packages/coverage/control.py", line 474, in _init_for_start
        self._init_data(suffix)
      File "/home/user/.local/lib/python3.8/site-packages/coverage/control.py", line 512, in _init_data
        ensure_dir_for_file(self.config.data_file)
      File "/home/user/.local/lib/python3.8/site-packages/coverage/misc.py", line 165, in ensure_dir_for_file
        ensure_dir(os.path.dirname(path))
      File "/.local/lib/python3.8/site-packages/coverage/misc.py", line 160, in ensure_dir
        os.makedirs(directory)
      File "/usr/lib/python3.8/os.py", line 223, in makedirs
        mkdir(name, mode)

Coverage.py predates sets as a built-in data structure, so the file data
collection has long been dicts with None as the values.  Sets are
available to us now (since Python 2.4 in 2004, which coverage.py dropped
support for in 2014!), we use sets.

Coverage config files supports 'command_line' parameters. However, only '-m' is parsed properly.

The line 'command_line = "--module unittest discover"' is considered as a path, not a module option.