ref-filter: fix stale parsed objects

ref-filter: fix stale parsed objects

In 054f5f457e (ref-filter: parse objects on demand, 2025-10-23) we have
started to skip parsing some objects in case we don't need to access
their values in the first place. This was done by introducing a new
member `struct expand_data::maybe_object` that gets populated on demand
via `get_or_parse_object()`.

This has led to a regression though where the object now gets reused
because we don't reset it properly. The `oi` structure is declared in
global scope, and there is no single place where we reset it before
invoking `get_object()`. The consequence is that the `maybe_object`
member doesn't get reset across calls, so subsequent calls will end up
reusing the same object.

This is only an issue for a subset of retrieved values, as not all of
the infrastructure ends up calling `get_or_parse_object()`. So the
effect is limited, which is probably why the issue wasn't detected
earlier.

Fix the issue by resetting `maybe_object` in `get_object()`.

Reported-by: Junio C Hamano <gitster@pobox.com>
Based-on-patch-by: Jeff King <peff@peff.net>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>