[git] master branch and 'origin/master' have diverged, how to 'undiverge' branches'?

In my case here is what I did to cause the diverged message: I did git push but then did git commit --amend to add something to the commit message. Then I also did another commit.

So in my case that simply meant origin/master was out of date. Because I knew no-one else was touching origin/master, the fix was trivial: git push -f (where -f means force)