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
Not sure if there is a way to get the actual diff format closer to Github style but $ git diff master...<branch_name> seems to show the set of changes that a pull request would show (assuming it's a pull request against master). The list of changed files in a pull request seems to be equivalent to $ git diff --name-status master...<branch_name>. I guess this all assumes that your local branches are up-to-date with the remote Github branches