The way you try to apply a diff to a different parent is by doing a three-way merge... the vast majority of tools do this by taking three files as arguments and

The way you try to apply a diff to a different parent is by doing a three-way merge... the vast majority of tools do this by taking three files as arguments and producing a fourth as output. The three-way merge is the underlying process which makes merge, rebase, cherry-pick, and revert work. They are all just "three-way merge, shuffle the arguments around, and adjust metadata". The parent + diff storage is not isomorphic to snapshot storage. Snapshot storage reflects the actual usage of VCS tools... people make changes, and record the final state. Parent + diff does not do this, it records the changes, which requires creating a diff, and there are multiple ways to create a diff between two snapshots. Git postpones the "which diff is correct" question until you actually care about the answer.