How do you recreate the GitHub pull request diff on the commandline?

The closest thing is to diff with the common ancestor between your feature-branch and your base-branch.

Something like:

git diff `git merge-base feature-branch base-branch`

If you don't want to do any git fetch and update a local PR branch, you simply can use cli/cli, the command-line interface for GitHub.

The release 0.9.0 includes gh pr diff