You can repeat step 4 of this recipe for different pull requests in the same repository if you want to test them simultaneously. If you are really happy with the result, you can create a branch to keep a reference to the result of the applied changes:
$ git checkout -b 12.0-custom
Using a different branch will help you remember that you are not using the version from GitHub, but a custom one.
The git branch command can be used to list all of the local branches you have in your repository.
From then on, if you need to apply the latest revision of the 12.0 branch from GitHub, you will need to pull it without using --ff-only:
$ git pull origin 12.0