How can I completely remove a file from a git repository?

This is what git filter-branch is for, but beware that your repo history will change, and the commit hashes will be different after history rewrite.

If you also want to free the space, I recommend that you use git forget-blob, because git filter-branch alone will not make git forget your file, as it may still be referenced by remotes, reflog, tags and such.

git forget-blob main.c.swp

You can get more information here

The tool you want is git filter-branch. Its usage is described here, but basically:

$ git filter-branch --tree-filter 'rm -f my_file' HEAD

will remove "my_file" from every commit.

Notice that this rewrites every commit, so if you push into a remote repository, you have to (a) force the update, and (b) everyone else who pulled from you will now have duplicate commits (since you rewrote the history), as described on the git rebase man page.

It's now recommended to use git-filter-repo instead. Running git-filter-branch actually prints:

WARNING: git-filter-branch has a glut of gotchas generating mangled history