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>

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