Merge branch 'hx/unpack-streaming'

Merge branch 'hx/unpack-streaming'

Allow large objects read from a packstream to be streamed into a
loose object file straight, without having to keep it in-core as a
whole.

* hx/unpack-streaming:
  unpack-objects: use stream_loose_object() to unpack large objects
  core doc: modernize core.bigFileThreshold documentation
  object-file.c: add "stream_loose_object()" to handle large object
  object-file.c: factor out deflate part of write_loose_object()
  object-file.c: refactor write_loose_object() to several steps
  unpack-objects: low memory footprint for get_data() in dry_run mode