[TortoiseSVN + PuTTY] Configure public key authentication

1. Create and save a PuTTY session where you specify the private key file in ppk format (PuTTYgen to convert from openssh format)
   
2. Checkout the repository using the PuTTY session name instead of the host name!
   
3. Use of PuTTY's plink: TortoiseSVN provides its own flavour plink programm which is the default. You can configure to use PuTTY's original but then you'll get an empty useless command line window which will automagically close on termination of the plink process.