You can do it like this:
In your repo,
git checkout -b new-branch
Then pull User A’s commits into your new-branch:
git pull git://github.com/[User A]/[project-name].git
After that, you can change it whatever you like in the new-branch. And when you test and satisfy with your changes, you can merge it into your master branch:
git checkout master
git merge new-branch
OK, now you have your code with User A and your changes.