"delta" of
another "base" object. These representations are assigned new types
ofs-delta and ref-delta, which is only valid in a pack file.

Both ofs-delta and ref-delta store the "delta" to be applied to
another object (called 'base object') to reconstruct the object. The
difference between them is, ref-delta directly encodes base object
name. If the base object is in the same pack, ofs-delta encodes
the offset of the base object in the pack instead.

The base object could also be deltified if it's in the same pack.
Ref-delta can also refer to an object outside the pack (i.e. the
so-called "thin pack"). When stored on disk however, the pack should
be self contained to avoid cyclic dependency.

The delta data starts with the size of the base object and the
size of the object to be reconstructed. These sizes are
encoded using the size encoding from above.  The remainder of
the delta data is a sequence of instructions to reconstruct the object
from the base object. If the base object is deltified, it must be
converted to canonical form first. Each instruction appends more and
more data to the target object until it's complete. There are two
supported instructions so far: one for copying a byte range from the
source object and one for inserting new data embedded in the
instruction itself.

Each instruction has variable length. Instruction type is determined
by the seventh bit of the first octet. The following diagrams follow
the convention in RFC 1951 (Deflate compressed data format).

==== Instruction to copy from base object

  +----------+---------+---------+---------+---------+-------+-------+-------+
  | 1xxxxxxx | offset1 | offset2 | offset3 | offset4 | size1 | size2 | size3 |
  +----------+---------+---------+---------+---------+-------+-------+-------+

This is the instruction format to copy a byte range from the source
object. It encodes the offset to copy from and the number of bytes to
copy. Offset and size are in little-endian order.

All offset and size bytes are optional. This is to reduce the
instruction size when encoding small offsets or sizes. The first seven
bits in the first octet determine which of the next seven octets is
present. If bit zero is set, offset1 is present. If bit one is set
offset2 is present and so on.

Note that a more compact instruction does not change offset and size
encoding. For example, if only offset2 is omitted like below, offset3
still contains bits 16-23. It does not become offset2 and contains
bits 8-15 even if it's right next to offset1.

  +----------+---------+---------+
  | 10000101 | offset1 | offset3 |
  +----------+---------+---------+

In its most compact form, this instruction only takes up one byte
(0x80) with both offset and size omitted, which will have default
values zero. There is another exception: size zero is automatically
converted to 0x10000.

==== Instruction to add new data

  +----------+============+
  | 0xxxxxxx |    data    |
  +----------+============+

This is the instruction to construct the target object without the base
object. The following data is appended to the target object. The first
seven bits of the first octet determine the size of data in
bytes. The size must be non-zero.

==== Reserved instruction

  +----------+============
  | 00000000 |
  +----------+============

This is the instruction reserved for future expansion.

== Original (version 1) pack-*.idx files have the following format:

  - The header consists of 256 4-byte network byte order
    integers.  N-th entry of this table records the number of
    objects in the corresponding pack, the first byte of whose
    object name is less than or equal to N.  This is called the
    'first-level fan-out' table.

  - The header is followed by sorted 24-byte entries, one entry
    per object in the pack.  Each entry is:

    4-byte network byte order integer, recording where the
    object is stored in the packfile as the offset from the
    beginning.

    one object name of the appropriate