submodule--helper: don't leak {run,capture}_command() cp.dir argument

submodule--helper: don't leak {run,capture}_command() cp.dir argument

Fix a memory leak in c51f8f94e5b (submodule--helper: run update
procedures from C, 2021-08-24) and 3c3558f0953 (submodule--helper: run
update using child process struct, 2022-03-15) by not allocating
memory in the first place.

The "dir" member of "struct child_process" will not be modified by
that API, and it's declared to be "const char *". So let's not
needlessly duplicate these strings.

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