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, and the pack
exclusion logic incorrectly skipped packs from layers above the
selected base, breaking reachability closure for bitmaps.
* 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