Fast-Forward per Merge Request
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### Description
We usually have two different types of merge-requests:
1. The MR contains multiple commits
2. The requester rebased everything into a nice single commit
So we don't want to enable fast-forward by default (for the first
case), but in the second case we get two more or less identical
commits.
### Proposal
For the second case we'd to like have a checkbox on the
merge-request creation form: to enable fast-forwarding for that
particular merge-request.
An idea we had: Can we detect if we are in the first or second case
and enable fast-forward automatically?
issue