The Git index file has the following format

All binary numbers are in network byte order. Version 2 is described
here unless stated otherwise.

Index entry

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.
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)
4-bit object type
  valid values in binary are 1000 (regular file), 1010 (symbolic link)
  and 1110 (gitlink)
3-bit unused
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.
160-bit SHA-1 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 pack-format.txt) 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.

Extensions

Cached tree

Cached tree extension contains pre-computed hashes for trees that can
be derived from the index. It helps speed up tree object generation
from index for a new commit.
When a path is updated in index, the path must be invalidated and
removed from tree cache.
The signature for this extension is { 'T', 'R', 'E', 'E' }.
A series of entries fill the entire extension; each of which
consists of:

Resolve undo

A conflict is represented in the index as a set of higher stage entries.
When a conflict is resolved (e.g. with "git add path"), these higher
stage entries will be removed and a stage-0 entry with proper resolution
is added.
When these higher stage entries are removed, they are saved in the
resolve undo extension, so that conflicts can be recreated (e.g. with
"git checkout -m"), in case users want to redo a conflict resolution
from scratch.
The signature for this extension is { 'R', 'E', 'U', 'C' }.
A series of entries fill the entire extension; each of which
consists of:

Split index

In split index mode, the majority of index entries could be stored
in a separate file. This extension records the changes to be made on
top of that to produce the final index.
The signature for this extension is { 'l', 'i', 'n', 'k' }.
The extension consists of: