list_objects_filter_copy(): deep-copy sparse_oid_name field

list_objects_filter_copy(): deep-copy sparse_oid_name field

The purpose of our copy function is to do a deep copy of each field so
that the source and destination structs become independent. We correctly
copy the filter_spec string list, but we forgot the sparse_oid_name
field. By doing a shallow copy of the pointer, that puts us at risk for
a use-after-free if one or both of the structs is cleaned up.

I don't think this can be triggered in practice, because we tend to leak
the structs rather than actually clean them up. But this should
future-proof us for plugging those leaks.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>