list-objects-filter-options: create copy helper

list-objects-filter-options: create copy helper

As we add more embedded members with type 'struct
list_objects_filter_options', it will be important to easily perform a
deep copy across multiple such structs. Create
list_objects_filter_copy() to satisfy this need.

This method is recursive to match the recursive nature of the struct.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>