By "unit of work", I mean the atomic delta which can, on its own, become part of the deployable state of the software. The thing which has a Change-Id in Gerrit.

The delta is the important thing. Git is deficient in this respect; it doesn't model a delta. Git hashes identify the tip of a tree.

When you rebase, you ought to be rebasing the change, the unit of work, a thing with an identity separate and independent of where it is based from.

And this is something that the jujutsu / Gerrit model fixes.