The '^' was the first syntax I used. I didn't understand why it wasn't working.
After experimenting with the options some more, I finally found my clue from the following error message from git:
"fatal: ambiguous argument 'staging': both revision and filename"
Fish was redirecting the stderr to a file with the name of my branch and so git was showing me just the log of the first branch instead of showing the difference.
Not sure if anything can/should be done about it, but thought I would report it anyway.