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

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