GitHub::Repositories::Repository update handler should check for "Visibility" property changes before attempting to update

[LucasQChen]

If your CFN template tries to update the "Visibility" property for GitHub::Repositories::Repository with the current value, the update will fail. For example, if your repo visibility is already private and your template specifies Visibility: Private, the update operation will fail.

Proposed fix: the handler code should contain a check to see if the Visibility property value needs to be changed before trying to execute the update.