It’s the fact that Mercurial tags the commit with the name of the branch that it was committed to which makes it much easier to determine whether a commit is included in your current branch or not.

Isn’t this trivial in Git too? git branch --contains COMMIT ?

Yep, NixOS as a base + some Flatpak store for installing apps. In fact, use impermanence to just drop all OS state apart from logs, network settings and flatpaks. That way, “turn it off and then on again” will almost always work to fix the OS.