Return an encoder for a basic signed varint value (does not include
  tag).

def _SignedVarintEncoder():
  """Return an encoder for a basic signed varint value (does not include
  tag)."""

  def EncodeSignedVarint(write, value):
    if value < 0:
      value += (1 << 64)
    bits = value & 0x7f
    value >>= 7
    while value:
      write(six.int2byte(0x80|bits))
      bits = value & 0x7f
      value >>= 7
    return write(six.int2byte(bits))

  return EncodeSignedVarint