Merge branch 'tb/bitmap-build-performance'

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()`