--  +--------------------------------+
fanout	    | fanout[0] = 2 (for example)    |-.
table	    +--------------------------------+ |
	    | fanout[1]                      | |
	    +--------------------------------+ |
	    | fanout[2]                      | |
	    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
	    | fanout[255] = total objects    |---.
	--  +--------------------------------+ | |
main	    | offset                         | | |
index	    | object name 00XXXXXXXXXXXXXXXX | | |
table	    +--------------------------------+ | |
	    | offset                         | | |
	    | object name 00XXXXXXXXXXXXXXXX | | |
	    +--------------------------------+<+ |
	  .-| offset                         |   |
	  | | object name 01XXXXXXXXXXXXXXXX |   |
	  | +--------------------------------+   |
	  | | offset                         |   |
	  | | object name 01XXXXXXXXXXXXXXXX |   |
	  | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~   |
	  | | offset                         |   |
	  | | object name FFXXXXXXXXXXXXXXXX |   |
	--| +--------------------------------+<--+
trailer	  | | packfile checksum              |
	  | +--------------------------------+
	  | | idxfile checksum               |
	  | +--------------------------------+
          .-------.
                  |
Pack file entry: <+

    packed object header:
1-byte size extension bit (MSB)
       type (next 3 bit)
       size0 (lower 4-bit)
       n-byte sizeN (as long as MSB is set, each 7-bit)
	size0..sizeN form 4+7+7+..+7 bit integer, size0
	is the least significant part, and sizeN is the
	most significant part.
    packed object data:
       If it is not DELTA, then deflated bytes (the size above
	is the size before compression).
If it is REF_DELTA, then
  base object name (the size above is the
	size of the delta data that follows).
         delta data, deflated.
If it is OFS_DELTA, then
  n-byte offset (see below) interpreted as a negative
	offset from the type-byte of the header of the
	ofs-delta entry (the size above is the size of
	the delta data that follows).
  delta data, deflated.

  offset encoding:
n bytes with MSB set in all but the last one.
The offset is then the number constructed by
concatenating the lower 7 bit of each byte, and
for n >= 2 adding 2^7 + 2^14 + ... + 2^(7*(n-1))
to the result.

have some other reorganizations.  They have the format:

4-byte signature:
    The signature is: {'M', 'I', 'D', 'X'}

1-byte version number:
    Git only writes or recognizes version 1.

1-byte Object Id Version
    We infer the length of object IDs (OIDs) from this value:
	1 => SHA-1
	2 => SHA-256
    If the hash type does not match the repository's hash algorithm,
    the multi-pack-index file should be ignored with a warning
    presented to the user.

1-byte number of "chunks"

1-byte number of base multi-pack-index files:
    This value is currently always zero.

4-byte number of pack files

(C + 1) * 12 bytes providing the chunk offsets:
    First 4 bytes describe chunk id. Value 0 is a terminating label.
    Other 8 bytes provide offset in current file for chunk to start.
    (Chunks are provided in file-order, so you can infer the length
    using the next chunk position if necessary.)

The CHUNK LOOKUP matches the table of contents from
the chunk-based file format, see gitformat-chunk[5].

The remaining data in the body is described one chunk at a time, and
these chunks may be given in any order. Chunks are required unless
otherwise specified.

Packfile Names (ID: {'P', 'N', 'A', 'M'})
    Store the names of packfiles as a sequence of NUL-terminated
    strings. There is no extra padding between the filenames,
    and they are listed in lexicographic order. The chunk itself
    is padded at the end with between 0 and 3 NUL bytes to make the
    chunk size a multiple of 4 bytes.

Bitmapped Packfiles (ID: {'B', 'T', 'M', 'P'})
    Stores a table of two 4-byte unsigned integers in network order.
    Each table entry corresponds to a single pack (in the order that
    they appear above in the `PNAM` chunk). The values for each table
    entry are as follows:
    - The first bit position (in pseudo-pack order, see below) to
      contain an object from that pack.
    - The number of bits whose objects are selected from that pack.

OID Fanout (ID: {'O', 'I', 'D', 'F'})
    The ith entry, F[i], stores the number of OIDs with first
    byte at most i. Thus F[255] stores the total
    number of objects.

OID Lookup (ID: {'O', 'I', 'D', 'L'})
    The OIDs for all objects in the MIDX are stored in lexicographic
    order in this chunk.

Object Offsets (ID: {'O', 'O', 'F', 'F'})
    Stores two 4-byte values for every object.
    1: The pack-int-id for the pack storing this object.
    2: The offset within the pack.
	If all offsets are less than 2^32, then the large offset chunk
	will not exist and offsets are stored as in IDX v1.
	If there is at least one offset value larger than 2^32-1, then
	the large offset chunk must exist, and offsets larger than
	2^31-1 must be stored in it instead. If the large offset chunk
	exists and the 31st bit is on, then removing that bit reveals
	the row in the large offsets containing the 8-byte offset of
	this object.

[Optional] Object Large Offsets (ID: {'L', 'O', 'F', 'F'})
    8-byte offsets into large packfiles.

[Optional] Bitmap pack order (ID: {'R', 'I', 'D', 'X'})
    A list of MIDX positions (one per object in the MIDX, num_objects in
    total, each a 4-byte unsigned integer in network byte order), sorted
    according to their relative bitmap/pseudo-pack positions.

Index checksum of the above contents.

|a,0|a,1|...|a,9|b,0|b,1|...|b,14|c,0|c,1|...|c,19|