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+Git commit graph format
+The Git commit graph stores a list of commit OIDs and some associated
+metadata, including:
+- The generation number of the commit. Commits with no parents have
+ generation number 1; commits with parents have generation number
+ one more than the maximum generation number of its parents. We
+ reserve zero as special, and can be used to mark a generation
+ number invalid or as "not computed".
+- The root tree OID.
+- The commit date.
+- The parents of the commit, stored using positional references within
+ the graph file.
+These positional references are stored as unsigned 32-bit integers
+corresponding to the array position withing the list of commit OIDs. We
+use the most-significant bit for special purposes, so we can store at most
+(1 << 31) - 1 (around 2 billion) commits.
+== Commit graph files have the following format:
+In order to allow extensions that add extra data to the graph, we organize
+the body into "chunks" and provide a binary lookup table at the beginning
+of the body. The header includes certain values, such as number of chunks
+and hash type.
+All 4-byte numbers are in network order.
+ 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 (1 = SHA-1)
+ We infer the hash length (H) from this value.
+ 1-byte number (C) of "chunks"
+ 1-byte (reserved for later use)
+ Current clients should ignore 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 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.
+ OID Fanout (ID: {'O', 'I', 'D', 'F'}) (256 * 4 bytes)
+ The ith entry, F[i], stores the number of OIDs with first
+ byte at most i. Thus F[255] stores the total
+ number of commits (N).
+ OID Lookup (ID: {'O', 'I', 'D', 'L'}) (N * H bytes)
+ The OIDs for all commits in the graph, sorted in ascending order.
+ Commit Data (ID: {'C', 'G', 'E', 'T' }) (N * (H + 16) bytes)
+ * The first H bytes are for the OID of the root tree.
+ * The next 8 bytes are for the positions of the first two parents
+ of the ith commit. Stores value 0xffffffff if no parent in that
+ position. If there are more than two parents, the second value
+ has its most-significant bit on and the other bits store an array
+ position into the Large Edge List chunk.
+ * The next 8 bytes store the generation number of the commit and
+ the commit time in seconds since EPOCH. The generation number
+ uses the higher 30 bits of the first 4 bytes, while the commit
+ time uses the 32 bits of the second 4 bytes, along with the lowest
+ 2 bits of the lowest byte, storing the 33rd and 34th bit of the
+ commit time.
+ Large Edge List (ID: {'E', 'D', 'G', 'E'}) [Optional]
+ This list of 4-byte values store the second through nth parents for
+ all octopus merges. The second parent value in the commit data stores
+ an array position within this list along with the most-significant bit
+ on. Starting at that array position, iterate through this list of commit
+ positions for the parents until reaching a value with the most-significant
+ bit on. The other bits correspond to the position of the last parent.
+ H-byte HASH-checksum of all of the above.