Apparent bug in 'git stash push <subdir>' loses untracked files

When running 'git stash push <subdir>' if there are both tracked and untracked files in this subdirectory, the tracked files are stashed but the untracked files are discarded.

I can reproduce this on my system (OSX, git 2.14.1) by running the below script as

    bash -x ./stashbug.sh &> output.txt

I could not find this indicated anywhere as an existing issue by performing generic searches, apologies if this is known.

  -Reid

Contents of stashbug.sh
------------------------
    #!/bin/sh

    uname -a
    git --version
    mkdir -p stashbug
    cd stashbug
    git init
    mkdir dir
    touch dir/tracked
    git add dir/tracked
    git commit -m 'initial'
    tree; git status
    mkdir dir/untracked_dir
    touch dir/untracked_dir/casualty1
    touch dir/casualty2
    echo 'contents' > dir/tracked
    tree; git status
    git stash push dir/
    git stash show -v
    tree; git status
    git stash pop
    tree; git status
------------------------

Resulting output.txt
---------------------
    + uname -a
    Darwin Reids-MacBook-Pro.local 15.6.0 Darwin Kernel Version
15.6.0: Tue Apr 11 16:00:51 PDT 2017;
root:xnu-3248.60.11.5.3~1/RELEASE_X86_64 x86_64
    + git --version
    git version 2.14.1
    + mkdir -p stashbug
    + cd stashbug
    + git init
    Initialized empty Git repository in /Users/reid/git/stashbug/.git/
    + mkdir dir
    + touch dir/tracked
    + git add dir/tracked
    + git commit -m initial
    [master (root-commit) 895197e] initial
     1 file changed, 0 insertions(+), 0 deletions(-)
     create mode 100644 dir/tracked
    + tree
    .
    └── dir
        └── tracked

    1 directory, 1 file
    + git status
    On branch master
    nothing to commit, working tree clean
    + mkdir dir/untracked_dir
    + touch dir/untracked_dir/casualty1
    + touch dir/casualty2
    + echo contents
    + tree
    .
    └── dir
        ├── casualty2
        ├── tracked
        └── untracked_dir
            └── casualty1

    2 directories, 3 files
    + git status
    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:   dir/tracked

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

        dir/casualty2
        dir/untracked_dir/

    no changes added to commit (use "git add" and/or "git commit -a")
    + git stash push dir/
    Saved working directory and index state WIP on master: 895197e initial
    + git stash show -v
    diff --git a/dir/tracked b/dir/tracked
    index e69de29..12f00e9 100644
    --- a/dir/tracked
    +++ b/dir/tracked
    @@ -0,0 +1 @@
    +contents
    + tree
    .
    └── dir
        └── tracked

    1 directory, 1 file
    + git status
    On branch master
    nothing to commit, working tree clean
    + 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:   dir/tracked

    no changes added to commit (use "git add" and/or "git commit -a")
    Dropped refs/stash@{0} (93ceee344b947ecd8a27a672e3aedd2b2e1acc99)
    + tree
    .
    └── dir
        └── tracked

    1 directory, 1 file
    + git status
    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:   dir/tracked

    no changes added to commit (use "git add" and/or "git commit -a")
---------------------