Operations: transform CloneOperation back to simple static method

[antis81]

@scunz Can you please review and confirm, that this matches your imagination? If so, I'll go on transforming the former CloneOperation, preparing it for our planned MGV-Service.

Things to do after the above is confirmed:

• transport the callbacks from Git::CloneOperation
• abstract git_clone_options to public Git::CloneOptions, making them available to the public API
  • Roughly spoken, the former Git::CloneOperation::setSomeCloneOption methods have to be extracted into a new public Git::CloneOptions class or struct.

[antis81]

@scunz Does this match to what you have in mind concerning the "service infrastructure"?

[scunz]

Yes, just running the clone without any threading is fine.