From https://stackoverflow.com/a/3801554 by PEdroArthur.
To list untracked files try:
git ls-files --others --exclude-standard
If you need to pipe the output to xargs, it is wise to mind white spaces using git ls-files -z and xargs -0:
git ls-files -z -o --exclude-standard | xargs -0 git add
For reference: git-ls-files