bugfixes:
- combine filter - ``[dict1, [dict2]] | combine`` now raise an error; previously ``combine`` had an undocumented behaviour where it was flattening the list before combining it (https://github.com/ansible/ansible/pull/57894#discussion_r339517518).
minor_changes:
- combine filter - now accept a ``list_merge`` argument which modifies its behaviour when the hashes to merge contain arrays/lists.
