Did I really just commit and push a file with a password in it?-Remove all traces of pushed Git commit

Surely I would never be so stupid as to commit a file that contains sensitive information — say my Oracle Cloud account’s private key. And even if I did, I would not push that change to a public git repo on GitHub, would I?

It turns out that I did exactly that. Ooops. Big one.

I quite frantically looked for a way to undo that mistake — remove the push and undo the commit as if it never had happened.