Re: [PATCH 7/8] combine-diff: Fast changed-to-all-parents paths scanning

> As was recently shown (c839f1bd "combine-diff: optimize
> combine_diff_path sets intersection"), combine-diff runs very slowly. In
> that commit we optimized paths sets intersection, but that accounted
> only for ~ 25% of the slowness, and as my tracing showed, for linux.git
> v3.10..v3.11, for merges a lot of time is spent computing
> diff(commit,commit^2) just to only then intersect that huge diff to
> almost small set of files from diff(commit,commit^1).
>
> That's because at present, to compute combine-diff, for first finding
> paths, that "every parent touches", we use the following combine-diff
> property/definition:
>
>     D(A,P1...Pn) = D(A,P1) ^ ... ^ D(A,Pn)      (w.r.t. paths)
>
> where
>
>     D(A,P1...Pn) is combined diff between commit A, and parents Pi
>
> and
>
>     D(A,Pi) is usual two-tree diff Pi..A