> git show --first-parent COMMIT # perspective of target branch = branch you were on when merging
> git show -m COMMIT # perspective of both branches
Lengthy explanation and more ways to do it: https://stackoverflow.com/questions/40986518/git-show-of-a-m...
Also the following works in my experience and is more easy to memorize because the command literally says what you want to know:
> git diff COMMIT_BEFORE_MERGE..MERGE_COMMIT