object-store: allow fetching objects via `has_object()`

object-store: allow fetching objects via `has_object()`

We're about to fully remove `repo_has_object_file()` in favor of
`has_object()`. The latter function does not yet have a way to fetch
missing objects via a promisor remote though, which means that it cannot
fully replace all usecases of `repo_has_object_file()`.

Introduce a new flag `HAS_OBJECT_FETCH_PROMISOR` that causes the
function to optionally fetch missing objects which are part of a
promisor pack. This flag will be used in the subsequent commit.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>