run-command: mark unused parameters in start_bg_wait callbacks

The start_bg_command() function takes a callback to tell when the background-ed process is "ready". The callback receives the child_process struct as well as an extra void pointer. But curiously, neither of the two users of this interface look at either parameter! This makes some sense. The only non-test user of the API is fsmonitor, which uses fsmonitor_ipc__get_state() to connect to a single global fsmonitor daemon (i.e., the one we just started!). So we could just drop these parameters entirely. But it seems like a pretty reasonable interface for the "wait" callback to have access to the details of the spawned process, and to have room for passing extra data through a void pointer. So let's leave these in place but mark the unused ones so that -Wunused-parameter does not complain. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>