The only way to purge that diff is with a force push. If multiple people are working on that same branch, you'd best let them know that you're rewriting history.
Assuming it was your last commit...
git reset --soft HEAD~
(undo password changes)
git diff
(make sure there are no changes that display the passwords)
(stage/commit changes)
git push origin +branch_name
Following standard security precautions, I would also reset those passwords if they're linked to sensitive data...