REF: https://stackoverflow.com/a/12681856
Untracked files are stored in the third parent of a stash commit. (This isn't actually documented, but is pretty obvious from The commit which introduced the -u feature, 787513..., and the way the rest of the documentation for git-stash phrases things... or just by doing git log --graph stash@{0})
You can view just the "untracked" portion of the stash via:
git show stash@{0}^3