I feel like this is much different. We're not saying "patch these things together into a materialized view", we're saying "here is the diff between your working directory and origin/master, which branch owns which hunk" and keeping that mapping around.

We keep monitoring it until we see changes upstream (someone merged into master) and then we let you update the difference (merging or rebasing commits you made if needed)

We're not inventing massively new mindspaces.