run_processes_parallel: change confusing task_cb convention

run_processes_parallel: change confusing task_cb convention

By declaring the task_cb parameter of type `void **`, the signature of
the get_next_task method suggests that the "task-specific cookie" can be
defined in that method, and the signatures of the start_failure and of
the task_finished methods declare that parameter of type `void *`,
suggesting that those methods are mere users of said cookie.

That convention makes a total lot of sense, because the tasks are pretty
much dead when one of the latter two methods is called: there would be
little use to reset that cookie at that point because nobody would be
able to see the change afterwards.

However, this is not what the code actually does. For all three methods,
it passes the *address* of pp->children[i].data.

As reasoned above, this behavior makes no sense. So let's change the
implementation to adhere to the convention suggested by the signatures.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Acked-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>