It is in the oacs-5-1 branch. It's just not tagged with openacs-5-1-compat, and won't be until the 5.1.2 release, unfortunately.
If you're working off the oacs-5-1 branch, you're fine! But if you're following the install guide, you are working off of the openacs-5-1-compat tag. And if you're upgrading via the repository, you are as well.