Actually clean on clean workspace, including hidden files (#71018)
Summary:
The workspace should be totally empty before checking out PyTorch; this
is especially important with non-ephemeral runners.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/71018
Reviewed By: robieta
Differential Revision: D33482985
Pulled By: suo
fbshipit-source-id: cafa123d2b893bfbdad62295586b5b79f1542b3a
