midx: teach `midx_fanout_add_midx_fanout()` about incremental MIDXs

The function `midx_fanout_add_midx_fanout()` is used to help construct
the fanout table when generating a MIDX by reusing data from an existing
MIDX.

Prepare this function to work with incremental MIDXs by making a few
changes:

  - The bounds checks need to be adjusted to start object lookups taking
    into account the number of objects in the previous MIDX layer (i.e.,
    by starting the lookups at position `m->num_objects_in_base` instead
    of position 0).

  - Likewise, the bounds checks need to end at `m->num_objects_in_base`
    objects after `m->num_objects`.

  - Finally, `midx_fanout_add_midx_fanout()` needs to recur on earlier
    MIDX layers when dealing with an incremental MIDX chain by calling
    itself when given a MIDX with a non-NULL `base_midx`.

Note that after 0c5a62f14b (midx-write.c: do not read existing MIDX with
`packs_to_include`, 2024-06-11), we do not use this function with an
existing MIDX (incremental or not) when generating a MIDX with
--stdin-packs, and likewise for incremental MIDXs.

But it is still used when adding the fanout table from an incremental
MIDX when generating a non-incremental MIDX (without --stdin-packs, of
course).

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

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