Why not just commit after the first step? You can then use the normal diff, and afterwards you just remove your temporary commit to make the final one.
First, a stash is a commit, just one that doesn't get in your way and that you don't have to keep track of yourself because it's automatically pointed to by a known reference called "stash".
Second, by using "git stash" to create this commit instead of "git commit", you save yourself the (small) burden of moving the commit out of your way and resetting the working tree back to the clean slate you started from – and upon which you want to try a new, slightly adjusted sledgehammer from scratch. That's exactly what "git stash" does in one step:
"Use git stash when you want to record the current state of the working directory and the index, but want to go back to a clean working directory." [1]
If you do that, you're just one accidental step away from uploading your bad changes to the origin. The stash way seems a bit safer and more natural to me.
