why: see https://stackoverflow.com/a/20653073/2249526
https://stackoverflow.com/a/29888735/2249526
If you already have checked out the code, the files are already indexed. After changing your git settings, say by running:
git config --global core.autocrlf input
you should refresh the indexes with
git rm --cached -r .
and re-write git index with
git reset --hard