list-objects-filter: initialize sub-filter structs

Since commit c54980ab83 (list-objects-filter: convert filter_spec to a strbuf, 2022-09-11), building with SANITIZE=undefined triggers an error in t5616. The problem is that we end up with a strbuf that has been zero-initialized instead of via STRBUF_INIT. Feeding that strbuf to strbuf_addbuf() in list_objects_filter_copy() means we will call memcpy like: memcpy(some_actual_buffer, NULL, 0); This works on most systems because we're copying zero bytes, but it is technically undefined behavior to ever pass NULL to memcpy. Even though c54980ab83 is where the bug manifests, that is only because we switched away from a string_list, which is OK with being zero-initialized (though it may cause other problems by not duplicating the strings, it happened to be OK in this instance). The actual bug is caused by the commit before that, 2a01bdedf8 (list-objects-filter: add and use initializers, 2022-09-11). There we consistently initialize the top-level filter structs, but we forgot the dynamically allocated ones we stick in filter_options->sub when creating combined filters. Note that we need to fix two spots here: where we parse a "combine:" filter, but also where we transform from a single-filter into a combined one after seeing multiple "--filter" options. In the second spot, we'll do some minor refactoring to avoid repeating our very-long array index. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>