What's the command to see the changelog that hasn't made it into the next minor release?

Exactly what the title says. I believe there’s a pre-existing script for this?

There is an unreleased.sh script in the repository. It accepts a git range, so for example to see all changes since 1.4.0, you can use the following:

./unreleased.sh v1.4.0..

Note that v1.4.0.. resolves to v1.4.0..HEAD, which may not be what you want if you’re working on stuff. Perhaps:

git fetch
./unreleased.sh v1.4.0..origin/master

Also worth mentioning that this is an internal tool, not subject to any kind of external guarantees of future support or compatibility.