Header:
  signature: Literal["Pack"]
  version
  object count
PACK:
  header
  object entry 1
  object entry 2
  ...
  pack checksum
object entry: full object entry | delta object entry

full object entry:
  type: blob/tree/commit/tag
  compressed_content: zlib(object content)

delta object entry:
  type: OFS_DELTA 或 REF_DELTA
  base: 基础对象
  compressed_delta: zlib(delta instructions)

delta instructions