Remove file from amended commit

Question

I pushed a commit to a repo where I accidentally added a file. Nobody else has merged from the remote repo so I can rewrite history. But when I remove file(unstage, not remove f

Answer 1

While I did something similar to what Colin and ydroneaud have suggested,

The answer was to use

git push +sa1:sa1

where sa1 is my branch. This forces to push even 'nothing'.