Merge branch 'tb/bitmap-build-performance' into seen

Reachability bitmap generation has been significantly optimized. By
reordering tree traversal, caching object positions, and refining how
pseudo-merge bitmaps are constructed, the performance of "git repack
--write-midx-bitmaps" is improved, especially for large repositories
and when using pseudo-merges.

* tb/bitmap-build-performance:
  pack-bitmap: build pseudo-merge bitmaps after regular bitmaps
  pack-bitmap: remember pseudo-merge parents
  pack-bitmap: sort bitmaps before XORing
  pack-bitmap: cache object positions during fill
  pack-bitmap: consolidate `find_object_pos()` success path
  pack-bitmap: reuse stored selected bitmaps
  pack-bitmap: check subtree bits before recursing
  pack-bitmap: pass object position to `fill_bitmap_tree()`