Commit Graphs Chains

--------------------

Typically, repos grow with near-constant velocity (commits per day). Over time,

the number of commits added by a fetch operation is much smaller than the

number of commits in the full history. By creating a "chain" of commit-graphs,

we enable fast writes of new commit data without rewriting the entire commit

history -- at least, most of the time.

## File Layout

A commit-graph chain uses multiple files, and we use a fixed naming convention

to organize these files. Each commit-graph file has a name

`$OBJDIR/info/commit-graphs/graph-{hash}.graph` where `{hash}` is the hex-

valued hash stored in the footer of that file (which is a hash of the file's

contents before that hash). For a chain of commit-graph files, a plain-text

file at `$OBJDIR/info/commit-graphs/commit-graph-chain` contains the

hashes for the files in order from "lowest" to "highest".

For example, if the `commit-graph-chain` file contains the lines

```

{hash0}

{hash1}

{hash2}

```

then the commit-graph chain looks like the following diagram:

+-----------------------+

| graph-{hash2}.graph |

+-----------------------+

|

+-----------------------+

| |

| graph-{hash1}.graph |

| |

+-----------------------+

|

+-----------------------+

| |

| |

| |

| graph-{hash0}.graph |

| |

| |

| |

+-----------------------+

Let X0 be the number of commits in `graph-{hash0}.graph`, X1 be the number of

commits in `graph-{hash1}.graph`, and X2 be the number of commits in

`graph-{hash2}.graph`. If a commit appears in position i in `graph-{hash2}.graph`,

then we interpret this as being the commit in position (X0 + X1 + i), and that

will be used as its "graph position". The commits in `graph-{hash2}.graph` use these

positions to refer to their parents, which may be in `graph-{hash1}.graph` or

`graph-{hash0}.graph`. We can navigate to an arbitrary commit in position j by checking

its containment in the intervals [0, X0), [X0, X0 + X1), [X0 + X1, X0 + X1 +

X2).