Git: checking out a file from a previous commit and amending it to HEAD

You’ve practically said it yourself:

First get the file back from one commit before:

$> git checkout HEAD~1 path/to/file.ext

Then commit it:

$> git commit -a -m 'Retrieved file from older revision'

If only the changes to that file where present in the last commit, you can even use git-revert:

$> git revert HEAD

I think it would be better to make this a separate commit, because it tells you exactly what you’ve reverted, and why. However, you can squash this into the previous commit by using the --amend switch to git-commit.