Show alert for user when fast-forward merge is required
Follow up of https://gitlab.com/gitlab-org/gitlab-ce/issues/39773
Do not allow user to click on merge button when fast-forward merge is required but not allowed. We display an error for CE but on EE we have a rebase button. Since we don't have the rebase feature for CE we should display the same warning with the merge button disabled.
MR needs attention of someone from front-end:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15258