Merge branch 'np/fast-import-idx-v2' into maint

* np/fast-import-idx-v2:
  fast-import: use the diff_delta() max_delta_size argument
  fast-import: honor pack.indexversion and pack.packsizelimit config vars
  fast-import: make default pack size unlimited
  fast-import: use write_idx_file() instead of custom code
  fast-import: use sha1write() for pack data
  fast-import: start using struct pack_idx_entry