* 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