Perhaps there is a better answer, but the best solution I’ve found so far is this.
First, you must control the definition of “whitespace” that Git is currently using.
git config core.whitespace '-trailing-space,-indent-with-non-tab,-tab-in-indent'
Next, you must control the definition of a word used. Instead of just using git diff -w, add --word-diff-regex='[^[:space:]]':
git diff -w --word-diff-regex='[^[:space:]]'
You’ll still see the context, which (in my case, since I’m trying to ensure that there are no differences except whitespace differences) is not helpful. You can use -U0 to tell Git to give you 0 lines of context, like so,
git diff -w -U0 --word-diff-regex='[^[:space:]]'
but you’ll still get output that looks pretty much like context, but it’s still much better than looking through all the changes carefully and manually to make sure they are only whitespace changes.
You can also do all the above in one command. The -c flag changes git config just for one command.
git -c core.whitespace=-trailing-space,-indent-with-non-tab,-tab-in-indent diff -U0 --word-diff-regex='[^[:space:]]'