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
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