Thanks, but I don't think that is quite what I want. What I mean by "cactus" history is that the feature branches have a linear history that starts at the tip of the master branch but when we merge them back into master we still use a merge commit. Looks like this:

      o-o-o   o-o-o   o-o
     /     \ /     \ /   \
    o-------o-------o-----o

We like doing that because, as the grandparent poster said, it clearly delineates the point where each PR was merged. AFAIK, the Gihub option to require a linear history assumes that you would want to flatten all the commits without any merge commits at all:

    o-o-o--o-o-o--o-o