Side question: when I look up at the tree corresponding to the stash (whether or not it's created using CLT or libgit2), I don't see my untracked file "text.txt" - why is that?

$ git cat-file -p stash@{0}^{tree}
100644 blob 90e5d6690018fc5a9c2d6b6ba61dde10d4e2acd6    hello_world.txt

$ git stash pop
On branch master
Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git checkout -- <file>..." to discard changes in working directory)

    modified:   hello_world.txt

Untracked files:
  (use "git add <file>..." to include in what will be committed)

    test.txt

Looking at the source code for git_stash_save() I see it is indeed creating 2 extra commits for the index and untracked files, so I understand where the topology observed in git objects is coming from.

Then the stash commit has 3 parents: these 2 commits and the base / HEAD commit. How are you supposed to reply these commits?

PS: Any plans to add a git_stash_apply() API?

I got this to work but I have the feeling I'm hacking this around and I'm not sure I'm doing this right:

1. Check if the stash commit has 3 parents and if so assume the 3rd one is the commit for the untracked files
2. If it exists, retrieve the tree and call git_checkout_tree()
3. Finally call git_checkout_tree() on the stash commit itself

The actual implementation of git apply [stash] is here:
https://github.com/git/git/blob/master/git-stash.sh#L440

See PR #2705.