Re: [PATCH 6/8] implement metadata cache subsystem

> There are some calculations that git makes repeatedly, even
> though the results are invariant for a certain input (e.g.,
> the patch-id of a certain commit). We can make a space/time
> tradeoff by caching these on disk between runs.
>
> Even though these may be immutable for a certain commit, we
> don't want to directly store the results in the commit
> objects themselves, for a few reasons:
>
>   1. They are not necessarily used by all algorithms, so
>      bloating the commit object might slow down other
>      algorithms.
>
>   2. Because they can be calculated from the existing
>      commits, they are redundant with the existing
>      information. Thus they are an implementation detail of
>      our current algorithms, and should not be cast in stone
>      by including them in the commit sha1.
>
>   3. They may only be immutable under a certain set of
>      conditions (e.g., which grafts or replace refs we are
>      using). Keeping the storage external means we can
>      invalidate and regenerate the cache whenever those
>      conditions change.