Allow squashing and fast-forwarding a merge request when a rebase is required

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See https://gitlab.com/gitlab-org/gitlab-ee/issues/895#note_20922445.

When the fast-forward merge method is selected, the rebase button will be shown if the source branch is not up-to-date with the target branch.

However, when we squash, we could just as easily apply the squashed commit on top of the target branch without a rebase, as long as there are no merge conflicts.

From the backend side, this isn't too difficult, but the UX is quite confusing. When the source branch isn't up-to-date with the target branch, you can:

1. Rebase the merge request. (Followed by a merge.)
2. Rebase and merge in one step.
3. Squash and merge, even if the MR is not rebase-able.