promisor-remote: document caller filtering contract

promisor_remote_get_direct() does not, on its happy path, filter out OIDs that are already present in the local object store: every OID the caller supplies is written to the fetch subprocess's stdin and ends up in the response pack. The only filtering it performs is in remove_fetched_oids(), and that only runs after a fetch failure when falling back to a different configured promisor remote. Almost every existing caller already filters locally-present OIDs out itself (typically with odb_read_object_info_extended() and OBJECT_INFO_FOR_PREFETCH, or odb_has_object() with no fetch flag). But the existing API comment does not state this expectation, so a new caller is easy to write incorrectly (I missed this originally and wrote two problematic callers). Omitting the filter still "works" in the sense that the desired objects end up local, but it silently makes the fetch request -- and the response pack -- larger than necessary, defeating part of the point of batching. Spell the contract out so future callers know to filter (and deduplicate) themselves, and point them at the helpers they should use to check local presence without accidentally triggering a lazy fetch. Signed-off-by: Elijah Newren <newren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>