Give maintainer freedom to chose merge or rebase

Proposal

We currently have these options how to handle merge request in our repositories

• Merge commit
• Merge commit with semi-linear history
• Fast-forward merge

It would be nice if a maintainer would be able to decide what to do with particular MR (merge, rebase), not to be forced to one option only.

At the end, I would be able to live with merge commit strategy, but in such case I'd like to prevent 'single commit' MRs with unnecessary additional merge commit. So option to do a fast forward if possible would be also nice.