pack-bitmap: handle duplicate pack entries during MIDX reuse

A MIDX pseudo-pack assigns one bit position to each selected OID. Its preferred-pack fast paths handle duplicate OIDs across packs in the MIDX, where the MIDX chooses one copy, but assume that each individual pack contains one entry per OID. Consider a preferred pack ordered [A, B, A, C]. The MIDX retains one copy of A, leaving three pseudo-pack positions for four physical entries. This creates two problems: - In MIDX-backed single-pack reuse, bitmap_nr came from pack->num_objects and therefore counted both copies of A. Read the selected count from the root MIDX layer BTMP chunk instead. If that layer has no BTMP chunk, skip pack reuse and let the ordinary packing path handle the bitmap-selected objects. MIDXs without BTMP lose preferred-pack reuse until rewritten, rather than requiring a scan of the entire pseudo-pack to retain an optional optimization. - Correcting the range length still does not align the two orderings. Once one copy of A is omitted, MIDX position N need not name physical pack entry N. That mismatch matters during both selection and output. Whole-word selection bypasses the per-object check that the exact physical base of a delta is present. Verbatim reuse copies the first N pack entries for the first N bits. The per-object writer likewise used each bit as a physical pack position. Use the direct mapping only when the range begins at zero and its selected count equals the physical entry count. Since selected entries remain in pack-offset order, equal counts mean that none was omitted and the two positions coincide. For every other MIDX range, let whole-word selection fall through to the existing per-bit path, which resolves the selected MIDX offset to a physical pack position before checking the delta base. Disable verbatim prefix reuse, and perform the same translation in the per-object writer. Leave classic single-pack bitmap handling unchanged. The production writer creates those bitmaps only for the pack it just wrote, which has one entry per OID. The test helper can target an existing pack, but rejects duplicate OIDs. Cover the A, B, A, C case in a MIDX-backed preferred pack. Check reuse with BTMP, then hide BTMP and check that the optional reuse path is skipped. Also make B a delta against the omitted copy of A and require normal packing to handle it. Signed-off-by: Taylor Blau <ttaylorr@openai.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>