I recently migrated a massive mono-repo from Phabricator to Github and sadly there is no equivalent to arc diff --preview
for Github. The best equivalent I've found to share your changes without actually creating a pull request / submitting your code for review is to create a new remote branch, push your changes there and share the link to your commit on the remote branch.
Say you're on master, this can be a simple flow
git checkout -b dev--preview
# make your changes on this branch
git add . && git commit -m "Previewing changes" && git push -u origin dev--preview
# copy the last commit id
git rev-parse HEAD | pbcopy
Your shareable link is going to be of this format
https://www.github.com/<org-name>/<repo-name>/commit/<your commit SHA from your clipboard>