You don't need to fork. I suggest using a different branch here. On this branch, you can add the new segment and when you think it is ready, merge that branch with your master branch. I don't think committing is a good idea here. If you use commits on your master branch and make some mistakes on your new segment, you might need to reverse some commits which might be difficult if you also commit other changes on your master branch. If you use a different branch and make a mistake, you just need to revert a commit on that branch and not on your master branch. And when the segment is finished, merge the new branch with your master branch.
Some information about using branches and merging: