About merge methods on GitHub→
You can allow contributors with push access to your repository to merge their pull requests on your GitHub Enterprise Server instance with different merge options or enforce a specific merge method for all of your repository's pull requests.
Configuring commit squashing for pull requests→
You can enforce, allow, or disable commit squashing for all pull request merges on your GitHub Enterprise Server instance in your repository.
Configuring commit rebasing for pull requests→
You can enforce, allow, or disable commit rebasing for all pull request merges on your GitHub Enterprise Server instance in your repository.