grep: use object_array_clear() in cmd_grep()

Free the "struct object_array" before exiting. This makes grep tests
(e.g.  "t7815-grep-binary.sh") a bit happer under SANITIZE=leak.

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

builtin/grep.c

@@ -1196,6 +1196,7 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 		run_pager(&opt, prefix);
 	clear_pathspec(&pathspec);
 	free_grep_patterns(&opt);
+	object_array_clear(&list);
 	free_repos();
 	return !hit;
 }