Merge branch 'ds/midx-with-less-memory'

The codepath to write multi-pack index has been taught to release a large chunk of memory that holds an array of objects in the packs, as soon as it is done with the array, to reduce memory consumption. * ds/midx-with-less-memory: write_midx_bitmap(): drop unused refs_snapshot parameter midx: reduce memory pressure while writing bitmaps midx: extract bitmap write setup pack-bitmap-write: use const for hashes