gitian: gitian-build.sh --setup complains if gitian.sigs etc already exist

[Sjors]

fatal: destination path 'gitian.sigs' already exists and is not an empty directory.
fatal: destination path 'bitcoin-detached-sigs' already exists and is not an empty directory.
fatal: destination path 'gitian-builder' already exists and is not an empty directory.

This contradicts the clone instructions here.

[maflcko]

Let me know if this is still an issue with the most recent version of gitian-build.py