Provide the option to use "git merge --ff-only" for pull requests (BB-7307)

I experimented with pull requests in my private repo. I did one commit into a feature branch, opened a pull request "feature->master" pressed "Merge". Master has not changed since feature branch start, so I expected "fast forward" merge to happen that would just append this one commit to master. I think this is default Git behavior – do fast-forward merge when it can. Instead, Bitbucket did an extra Merge commit, having previous master tip and my branch commit as parents.

I think linear history is easier to read. It would be good to have fast forward merging by default when possible, or at least have an option to do so. To achieve this currently I have to merge this branch locally and then pushing to master, avoiding the whole "pull request" feature.

Or are pull requests intended to behave like this? If so, why?