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>

builtin/ls-files.c

@@ -781,6 +781,7 @@ int cmd_ls_files(int argc, const char **argv, const char *cmd_prefix)
 		ret = 1;
 	}
 
+	string_list_clear(&exclude_list, 0);
 	dir_clear(&dir);
 	free(max_prefix);
 	return ret;

t/t3001-ls-files-others-exclude.sh

@@ -8,6 +8,7 @@ test_description='git ls-files --others --exclude
 This test runs git ls-files --others and tests --exclude patterns.
 '
 
+TEST_PASSES_SANITIZE_LEAK=true
 . ./test-lib.sh
 
 rm -fr one three
@@ -102,7 +103,7 @@ test_expect_success \
        >output &&
      test_cmp expect output'
 
-test_expect_success 'restore gitignore' '
+test_expect_success !SANITIZE_LEAK 'restore gitignore' '
 	git checkout --ignore-skip-worktree-bits $allignores &&
 	rm .git/index
 '
@@ -125,7 +126,7 @@ cat > expect << EOF
 #	three/
 EOF
 
-test_expect_success 'git status honors core.excludesfile' \
+test_expect_success !SANITIZE_LEAK 'git status honors core.excludesfile' \
 	'test_cmp expect output'
 
 test_expect_success 'trailing slash in exclude allows directory match(1)' '