submodule--helper: fix "reference" leak

submodule--helper: fix "reference" leak

Fix leaks in the "reference" variable declared in add_submodule() and
module_clone().

In preceding commits this variable was refactored out of the "struct
module_clone_data", but the leak has been with us since
31224cbdc72 (clone: recursive and reference option triggers submodule
alternates, 2016-08-17) and 8c8195e9c3e (submodule--helper: introduce
add-clone subcommand, 2021-07-10).

Those commits added an xstrdup()'d member of the
STRING_LIST_INIT_NODUP'd "struct string_list". We need to free()
those, but not the ones we get from argv, let's make use of the "util"
member, if it has a pointer it's the pointer we'll need to free,
otherwise it'll be NULL (i.e. from argv).

Note that the free() of the "util" member is needed in both
module_clone() and add_submodule(). The module_clone() function itself
doesn't populate the "util" pointer as add_submodule() does, but
module_clone() is upstream of the
add_possible_reference_from_superproject() caller we're modifying
here, which does do that.

This does preclude the use of the "util" pointer for any other reasons
for now, but that's OK. If we ever need to use it for something else
we could turn it into a small "struct" with an optional "to_free"
member, and switch to using string_list_clear_func().

Alternatively we could have another "struct string_list to_free" which
would keep a copy of the strings we've dup'd to free(). But for now
this is perfectly adequate.

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>