ls-files: add missing string_list_clear()

ls-files: add missing string_list_clear()

Fix a memory leak that's been here ever since 72aeb18772d (clean.c,
ls-files.c: respect encapsulation of exclude_list_groups, 2013-01-16),
we dup'd the argument in option_parse_exclude(), but never freed the
string_list.

This makes almost all of t3001-ls-files-others-exclude.sh pass (it had
a lot of failures before). Let's mark it as passing with
TEST_PASSES_SANITIZE_LEAK=true, and then exclude the tests that still
failed with a !SANITIZE_LEAK prerequisite check until we fix those
leaks. We can still see the failed tests under
GIT_TEST_FAIL_PREREQS=true.

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