Add the option to reverse merge when a merge request is out-of-date

Description

Currently it is possible to restrict merges to fast-forward merges, requiring a rebase before the merge if this is not the case. https://docs.gitlab.com/ee/user/project/merge_requests/#semi-linear-history-merge-requests

Proposal

There should also be an option to update the branch by reverse merging from the target branch to the source branch as some people like to keep their git history untouched.

Links / references

Based on gitlab-ce#42554