midx: teach `nth_midxed_object_oid()` about incremental MIDXs

midx: teach `nth_midxed_object_oid()` about incremental MIDXs

The function `nth_midxed_object_oid()` returns the object ID for a given
object position in the MIDX lexicographic order.

Teach this function to instead operate over the concatenated
lexicographic order defined in an earlier step so that it is able to be
used with incremental MIDXs.

To do this, we need to both (a) adjust the bounds check for the given
'n', as well as record the MIDX-local position after chasing the
`->base_midx` pointer to find the MIDX which contains that object.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>