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