PSA that you ought to use difftastic for your diffs:
https://difftastic.wilfred.me.uk/
It's an easy install and the resulting output is honestly great.
(You can still get the old diff output when necessary by running e.g. `jj diff --git` or `git diff --no-ext-diff`.)
Also here's how to configure it with jj such that it properly diffs across file renames:
https://github.com/jj-vcs/jj/discussions/6462