run-command.c: don't copy "data" to "struct parallel_processes"

run-command.c: don't copy "data" to "struct parallel_processes"

As with the *_fn members removed in a preceding commit, let's not copy
the "data" member of the "struct run_process_parallel_opts" over to
the "struct parallel_processes". Now that we're passing the "opts"
down there's no reason to do so.

This makes the code easier to follow, as we have a "const" attribute
on the "struct run_process_parallel_opts", but not "struct
parallel_processes". We do not alter the "ungroup" argument, so
storing it in the non-const structure would make this control flow
less obvious.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>