Git: Performing a diff between two stashes
As mentioned by Confuzing and Per Quested Aronsson in the comments, the simplest solution is:
git diff stash@{2} stash@{3}
It also works with the -p added, but that does not add much value.
git diff -p stash@{2} -p stash@{3}
Oddly, Git's documentation does not suggest to use twice the -p option. I just tried it and found out it worked.