After doing a \"simple\" merge (one without conflicts), git show usually only shows something like
git show
commit 0e1329e551a5700614a2a34d8101e92fd9f2ca
If your merge commit is commit 0e1329e5, as above, you can get the diff that was contained in this merge by:
git diff 0e1329e5^..0e1329e5
I hope this helps!