Prevent os.walk() scanning excluded dirs.

[bwoodsend]

Rather than discovering every file then eliminating those in excluded folders, this eliminates searching the excluded folders to begin with.

This is a big speedup if you have an exlcuded virtual environment sat in a repository.

[coveralls]

Coverage decreased (-0.02%) to 94.483% when pulling e6c0c02 on bwoodsend:skip-walking-excluded-dirs into 503bc40 on google:main.

[bwendling]

Thank you!