You can set "upstream" to another local branch, no remote is required, it's just how it's typically used. Under the hood it's just a way of naming another reference as a divergence point.
I use this extensively with Git(Hub|Lab) (PR|MR)'s. I set the upstream to "master" but push to "topic". The "push.default" docs cover how to have your cake and eat it too there.
> You'd open an editor with A..D with that, whereas you perhaps only want B..D?
That wouldn't be much of an issue for me, but I still tried to implement an option that doesn't rely on my repository having a defined remote.
I quite often start a new local git repository whenever I'm about to start a new experiment (or just scripting some one-off task), and most of these will never get pushed onto a git server. However, even in those cases, my scatterbrain often triggers the "gah, that should have been in a previous commit!" reflex.
By the way: Thank you very much for taking your time and reading all this! :)
Edit: I just re-read the upstream-point thing. Wouldn't that cause a `git push` on `thing` to push directly to `origin/master`?
That also wouldn't be compatible with the workflow we have at work (we use GitLab merge requests)…