With git, why does `rbt post` send a cumulative diff of the branch heads?

[Michael van der Kamp]

I noticed that `rbt post` sends a cumulative diff along with the individual commit diffs, and this diff is generated using the syntax `git diff A..B`. I have a couple questions:

1. What is the purpose of this cumulative diff?
2. This generates a diff between the current state of `A` against the current state of `B`, which can extremely large if `A` has progressed since `B` was branched off of it. Why is this needed? Why not just compare against the merge-base of `A` and `B`, e.g. by doing `git diff A...B` instead?

Cheers!

[Michael van der Kamp]

Nevermind, I found https://groups.google.com/g/reviewboard/c/_drAGy5jhzs/m/oKWCGdlwEQAJ