I learned today about the -v (--verbose) flag to git commit (git-commit), which causes a unified diff of what would be committed to be appended to the end of the commit message. This diff is not part of the commit. Set the commit.verbose configuration variable (new in Git 2.9) to adjust the default behavior.
I also learned about using git show (git-show) to display the diff for the most recent commit. I had been using git log -1 --patch (git-log). More on git log -p vs. git show vs. git diff.