4-byte signature:
    The signature is: {'C', 'G', 'P', 'H'}

1-byte version number:
    Currently, the only valid version is 1.

 1-byte Hash Version
     We infer the hash length (H) from this value:
1 => SHA-1
2 => SHA-256
     If the hash type does not match the repository's hash algorithm, the
     commit-graph file should be ignored with a warning presented to the
     user.

1-byte number (C) of "chunks"

1-byte number (B) of base commit-graphs
    We infer the length (H*B) of the Base Graphs chunk
    from this value.

(C + 1) * 12 bytes listing the table of contents for the chunks:
    First 4 bytes describe the chunk id. Value 0 is a terminating label.
    Other 8 bytes provide the byte-offset in current file for chunk to
    start. (Chunks are ordered contiguously in the file, so you can infer
    the length using the next chunk position if necessary.) Each chunk
    ID appears at most once.

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.

H-byte HASH-checksum of all of the above.