For this query, there is an update in git so for
New versions of git now have
git stash --all
which stashes all files, including untracked and ignored files.
git stash --include-untracked
no longer touches ignored files.
As of version 1.7.7, you can use
git stash --include-untracked
or
git stash save -u
to stash untracked files without staging them.
Add (git add) the file and start tracking it. Then stash. But with the update, this is no longer exists.
Reference: https://git-scm.com/docs/git-stash