master branch and ‘origin/master’ have diverged, how to ‘undiverge’ branches’?

Somehow my master and my origin/master branch have diverged.
I actually don’t want them to diverge.

How can I view these differences and merge them?

I had this and am mystified as to what has caused it, even after reading the above responses. My solution was to do

git reset --hard origin/master

Then that just resets my (local) copy of master (which I assume is screwed up) to the correct point, as represented by (remote) origin/master.

WARNING: You will lose all changes not yet pushed to origin/master.