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>