Unfortunately I accidentally pushed an incorrect v3.1.2 tag yesterday. I immediately recognised that it was wrong, so quickly deleted it and pushed the correct tag. I thought that this would not be a problem for anyone who was not pulling the rails repository at that exact moment.
It turns out I was wrong. If you have a rails repository clone that existed before the 3.1.2 release, in order to get the v3.1.2 tag into your repository, you will need to do:
git fetch origin tag v3.1.2
I am very sorry for the inconvenience.