Included in GitLab Enterprise Edition Starter.
If you prefer a linear Git history and a way to accept merge requests without creating merge commits, you can configure this on a per-project basis.
Navigate to the project's settings, select the Fast-forward merge option and hit Save changes for the changes to take effect.
Now, when you visit the merge request page, you will be able to accept it only if a fast-forward merge is possible.
If the target branch is ahead of the source branch, you need to rebase the source branch before you will be able to do a fast-forward merge.