After doing a \"simple\" merge (one without conflicts), git show usually only shows something like
git show
commit 0e1329e551a5700614a2a34d8101e92fd9f2ca
If you are sitting at the merge commit then this shows the diffs:
git diff HEAD~1..HEAD
If you're not at the merge commit then just replace HEAD with the merge commit. This method seems like the simplest and most intuitive.