Merge branch 'tb/midx-incremental-custom-base' into seen

The 'git multi-pack-index write --incremental' command has been corrected to properly honor the '--base' option. Previously, the custom base was ignored by the normal write path; packs from layers above the selected base were incorrectly skipped by the pack exclusion logic, and reachability closure for bitmaps was broken. * tb/midx-incremental-custom-base: midx-write: include packs above custom incremental base midx: pass custom '--base' through incremental writes t5334: expose shared `nth_line()` helper