Merge branch 'ac/bitmap-lookup-table'

The pack bitmap file gained a bitmap-lookup table to speed up locating the necessary bitmap for a given commit. * ac/bitmap-lookup-table: pack-bitmap-write: drop unused pack_idx_entry parameters bitmap-lookup-table: add performance tests for lookup table pack-bitmap: prepare to read lookup table extension pack-bitmap-write: learn pack.writeBitmapLookupTable and add tests pack-bitmap-write.c: write lookup table extension bitmap: move `get commit positions` code to `bitmap_writer_finish` Documentation/technical: describe bitmap lookup table extension