Re: [PATCH v3 5/6] commit-graph: implement file format version 2

> 
> OK, let's try that then, on top of this series:
> 
>     diff --git a/commit-graph.c b/commit-graph.c
>     index 5eebba6a0f..36c8cdb950 100644
>     --- a/commit-graph.c
>     +++ b/commit-graph.c
>     @@ -1127,7 +1127,7 @@ int write_commit_graph(const char *obj_dir,
>             case 2:
>                     hashwrite_u8(f, num_chunks);
>                     hashwrite_u8(f, 1); /* reachability index version */
>     -               hashwrite_u8(f, 0); /* unused padding byte */
>     +               hashwrite_u8(f, 1); /* unused padding byte */
> 
> Then:
> 
>     $ ~/g/git/git --exec-path=$PWD commit-graph write --version=2; ~/g/git/git --exec-path=$PWD status
>     Expanding reachable commits in commit graph: 100% (201645/201645), done.
>     Computing commit graph generation numbers: 100% (200556/200556), done.
>     error: unsupported value in commit-graph header
>     HEAD detached at pr-112/derrickstolee/graph/v2-head-v3
> 
> So we'll error out in the same way as if "2.0" was changed to "3.0" with
> this "2.1" change, just with a more cryptic error message on this new v2
> code.
> 
> I don't see how this is meaningfully different from just bumping the
> version to "3". We abort parsing the graph just like with major version
> changes.