How do you fix a bad merge, and replay your good commits onto a fixed merge?

Question

I accidentally committed an unwanted file (filename.orig while resolving a merge) to my repository several commits ago, without me noticing it until now. I want

Answer 1

You should probably clone your repository first.

Remove your file from all branches history:
git filter-branch --tree-filter 'rm -f filename.orig' -- --all

Remove your file just from the current branch:
git filter-branch --tree-filter 'rm -f filename.orig' -- --HEAD    

Lastly you should run to remove empty commits:
git filter-branch -f --prune-empty -- --all