I can enforce a merge strategy per project, but we need to change the merge strategy at the branch level. We can allow multiple strategies, but the person who does the merge has to be diligent in selecting the correct one. This is most problematic when merging a long-running development branch into our master branch. We squash PR's to the development branch, but we don't want the PR from development to master to be squashed. Relying on the person who is charged with merging from development to master to make that proper choice has not worked out so well for us.
How can we enforce a merge strategy on a per-branch basis?