odb: convert `odb_has_object()` flags into an enum

odb: convert `odb_has_object()` flags into an enum

Following the reason in the preceding commit, convert the
`odb_has_object()` flags into an enum.

With this change, we would have catched the misuse of `odb_has_object()`
that was fixed in a preceding commit as the compiler would have
generated a warning:

  ../builtin/backfill.c:71:9: error: implicit conversion from enumeration type 'enum odb_object_info_flag' to different enumeration type 'enum odb_has_object_flag' [-Werror,-Wenum-conversion]
     70 |                 if (!odb_has_object(ctx->repo->objects, &list->oid[i],
        |                      ~~~~~~~~~~~~~~
     71 |                                     OBJECT_INFO_FOR_PREFETCH))
        |                                     ^~~~~~~~~~~~~~~~~~~~~~~~
  1 error generated.

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