Looking for help to understand external filter driver code

Hi,

a long time ago in aa4ed4 Junio introduced the external filter driver definition. Since that time we fork the Git process and then we fork again to run the external filter. This is probably a super stupid question with an obvious answer... but can anyone help me to understand the code and explain why we fork twice? Wouldn't it be sufficient to just fork once for the external filter since we cannot process anything in parallel anyways?

In 546bb5 Hannes refactored Junio's code to use the async infrastructure that is used today.

Thank you, Lars