git add path/to/untracked-file
git stash

git stash --include-untracked
git stash -u

git add path/to/untracked-file
git stash

git add -N path/to/untracked/file     # note: -N is short for --intent-to-add
git stash

path/to/untracked-file: not added yet
fatal: git-write-tree: error building trees
Cannot save the current index state

git add path/to/untracked-file
git stash save "don't forget to un-add path/to/untracked-file" # stash w/reminder
# do some other work
git stash list
# shows:
# stash@{0}: On master: don't forget to un-add path/to/untracked-file
git stash pop   # or apply instead of pop, to keep the stash available
git rm --cached path/to/untracked-file

git ls-files -o > files-to-untrack
git add `cat files-to-untrack` # note: files-to-untrack will be listed, itself!
git stash
# do some work
git stash pop
git rm --cached `cat files-to-untrack`
rm files-to-untrack

alias stashall='git ls-files -o > .gftu; git add `cat .gftu`; git stash'
alias unstashall='git stash pop; git rm --cached `cat .gftu`; rm .gftu'

function unstashall(){git stash "${@:-pop}";git rm --cached `cat .gftu`;rm .gftu}

git add --intent-to-add path/to/untracked-file

git update-index --add --cacheinfo 100644 e69de29bb2d1d6434b8b29ae775ad8c2e48c5391 path/to/untracked-file

$ git stash
b.rb: not added yet
fatal: git-write-tree: error building trees
Cannot save the current index state