I have Difftastic configured as my default git diff tool with this gitconfig entry:

[diff]
	external = difft

This causes git diff REF to output in a different format which in turn breaks the --since flag functionality which relies on the output to be in an expected format.

I was hoping git diff might have a flag to force the default diff tool for a single command but as far as I can tell that isn't the case.

One solution for me would be to just revert to using the regular git diff output but I'd rather not do this so I'm opening an issue hoping you have a better idea. I wonder if prioritising #1337 would make this issue go away or if that would still need to start with parsing a git diff output?