Bring rebase and fast forward-merge to CE

• Bring rebase and fast forward-merge to CE
• Same exact design and implementation as the feature currently in EE, i.e. https://docs.gitlab.com/ee/user/project/merge_requests/fast_forward_merge.html

• Not that we are not bringing squash and merge to CE, i.e. https://docs.gitlab.com/ee/user/project/merge_requests/squash_and_merge.html is not in scope.