Usually, to discard changes to a file you would do:
git checkout --
What if the change I want to discard is deleting the file?
Assuming you're wanting to undo the effects of git rm or rm followed by git add -A or something similar:
# this restores the file status in the index
git reset --
# then check out a copy from the index
git checkout --
To undo git add , the first line above suffices, assuming you haven't committed yet.