In the Magit Refs buffer, you can press tab on a branch (the branch I want to merge in) to see what commits would be added if this branch were merged into the currently checked out branch, and you can press enter on the commits to see their changes. Is there a way to see all those changes unified together?
I don't think the magit diff dwim feature is what I want, because it shows all the changes that are on the current branch but not on the branch I want to merge in. I usually don't care about seeing those changes, because I am mostly interested in what the branch I am merging will change, not everything else that changed.