All binary numbers are in network byte order.
In a repository using the traditional SHA-1, checksums and object IDs
(object names) mentioned below are all computed using SHA-1.  Similarly,
in SHA-256 repositories, these values are computed using SHA-256.
Version 2 is described here unless stated otherwise.

Index entries are sorted in ascending order on the name field,
interpreted as a string of unsigned bytes (i.e. memcmp() order, no
localization, no special casing of directory separator '/'). Entries
with the same name are sorted by their stage field.

An index entry typically represents a file. However, if sparse-checkout
is enabled in cone mode (`core.sparseCheckoutCone` is enabled) and the
`extensions.sparseIndex` extension is enabled, then the index may
contain entries for directories outside of the sparse-checkout definition.
These entries have mode `040000`, include the `SKIP_WORKTREE` bit, and
the path ends in a directory separator.

32-bit ctime seconds, the last time a file's metadata changed
  this is stat(2) data

32-bit ctime nanosecond fractions
  this is stat(2) data

32-bit mtime seconds, the last time a file's data changed
  this is stat(2) data

32-bit mtime nanosecond fractions
  this is stat(2) data

32-bit dev
  this is stat(2) data

32-bit ino
  this is stat(2) data

32-bit mode, split into (high to low bits)

16-bit unused, must be zero

4-bit object type
  valid values in binary are 1000 (regular file), 1010 (symbolic link)
  and 1110 (gitlink)

3-bit unused, must be zero

9-bit unix permission. Only 0755 and 0644 are valid for regular files.
Symbolic links and gitlinks have value 0 in this field.

32-bit uid
  this is stat(2) data

32-bit gid
  this is stat(2) data

32-bit file size
  This is the on-disk size from stat(2), truncated to 32-bit.

Object name for the represented object

A 16-bit 'flags' field split into (high to low bits)

1-bit assume-valid flag

1-bit extended flag (must be zero in version 2)

2-bit stage (during merge)

12-bit name length if the length is less than 0xFFF; otherwise 0xFFF
is stored in this field.

(Version 3 or later) A 16-bit field, only applicable if the
"extended flag" above is 1, split into (high to low bits).

1-bit reserved for future

1-bit skip-worktree flag (used by sparse checkout)

1-bit intent-to-add flag (used by "git add -N")

13-bit unused, must be zero

Entry path name (variable length) relative to top level directory
  (without leading slash). '/' is used as path separator. The special
  path components ".", ".." and ".git" (without quotes) are disallowed.
  Trailing slash is also disallowed.

The exact encoding is undefined, but the '.' and '/' characters
are encoded in 7-bit ASCII and the encoding cannot contain a NUL
byte (iow, this is a UNIX pathname).

(Version 4) In version 4, the entry path name is prefix-compressed
  relative to the path name for the previous entry (the very first
  entry is encoded as if the path name for the previous entry is an
  empty string).  At the beginning of an entry, an integer N in the
  variable width encoding (the same encoding as the offset is encoded
  for OFS_DELTA pack entries; see gitformat-pack[5]) is stored, followed
  by a NUL-terminated string S.  Removing N bytes from the end of the
  path name for the previous entry, and replacing it with the string S
  yields the path name for this entry.

1-8 nul bytes as necessary to pad the entry to a multiple of eight bytes
while keeping the name NUL-terminated.

(Version 4) In version 4, the padding after the pathname does not
exist.

Interpretation of index entries in split index mode is completely
different. See below for details.

Untracked cache saves the untracked file list and necessary data to
verify the cache. The signature for this extension is { 'U', 'N',
'T', 'R' }.

The extension starts with

The file system monitor cache tracks files for which the core.fsmonitor
hook has told us about changes.  The signature for this extension is
{ 'F', 'S', 'M', 'N' }.

The extension starts with

The End of Index Entry (EOIE) is used to locate the end of the variable
length index entries and the beginning of the extensions. Code can take
advantage of this to quickly locate the index extensions without having
to parse through all of the index entries.

Because it must be able to be loaded before the variable length cache
entries and other index extensions, this extension must be written last.
The signature for this extension is { 'E', 'O', 'I', 'E' }.

The extension consists of:

The Index Entry Offset Table (IEOT) is used to help address the CPU
cost of loading the index by enabling multi-threading the process of
converting cache entries from the on-disk format to the in-memory format.
The signature for this extension is { 'I', 'E', 'O', 'T' }.

The extension consists of:

When using sparse-checkout in cone mode, some entire directories within
the index can be summarized by pointing to a tree object instead of the
entire expanded list of paths within that tree. An index containing such
entries is a "sparse index". Index format versions 4 and less were not
implemented with such entries in mind. Thus, for these versions, an
index containing sparse directory entries will include this extension
with signature { 's', 'd', 'i', 'r' }. Like the split-index extension,
tools should avoid interacting with a sparse index unless they understand
this extension.