Help shows silly program name when running coverage as a module

[nedbat]

$ python -m coverage
Code coverage for Python.  Use '__main__.py help' for help.

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• Bitbucket: https://bitbucket.org/ned/coveragepy/issue/478

[nedbat]

Scott Belden fixed this in a GitHub pull request that I applied in 48e6d2a9b362 (bb).