Re: [PATCH 5/7] commit-graph: document file format v2

> From: Derrick Stolee <derrickstolee@github.com>
>
> The corrected commit date was first documented in 5a3b130ca (doc: add
> corrected commit date info, 2021-01-16) and it used an optional chunk to
> augment the commit-graph format without modifying the file format
> version.
>
> One major benefit to this approach is that corrected commit dates could
> be written without causing a backwards compatibility issue with Git
> versions that do not understand them. The topological level was still
> available in the CDAT chunk as it was before.
>
> However, this causes a different issue: more data needs to be loaded
> from disk when parsing commits from the commit-graph. In cases where
> there is no significant algorithmic gain from using corrected commit
> dates, commit walks take up to 20% longer because of this extra data.
>
> Create a new file format version for the commit-graph format that
> differs only in the CDAT chunk: it now stores corrected commit date
> offsets. This brings our data back to normal and will demonstrate
> performance gains in almost all cases.