The merge button on pull request does "git merge -no-ff".
This is highly undesirable as it gives a bushy history line for no good reason.
As it is not configurable at the moment, at least it should simply call "git merge" which allow fast forwards.
Better yet merge behaviour and strategy should be configurable for the repository with the same default as "git merge" itself.
And/or even allow these options for each pull request merge.