midx: teach `midx_preferred_pack()` about incremental MIDXs

The function `midx_preferred_pack()` is used to determine the identity
of the preferred pack, which is the identity of a unique pack within
the MIDX which is used as a tie-breaker when selecting from which pack
to represent an object that appears in multiple packs within the MIDX.

Historically we have said that the MIDX's preferred pack has the unique
property that all objects from that pack are represented in the MIDX.
But that isn't quite true: a more precise statement would be that all
objects from that pack *which appear in the MIDX* are selected from that
pack.

This helps us extend the concept of preferred packs across a MIDX chain,
where some object(s) in the preferred pack may appear in other packs
in an earlier MIDX layer, in which case those object(s) will not appear
in a subsequent MIDX layer from either the preferred pack or any other
pack.

Extend the concept of preferred packs by using the pack which represents
the object at the first position in MIDX pseudo-pack order belonging to
the current MIDX layer (i.e., at position 'm->num_objects_in_base').

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

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