Bug/regression report - 'git stash push -u' fatal errors when sub-repo present

Thank you for filling out a Git bug report! Please answer the following questions to help us understand your issue.

What did you do before the bug happened? (Steps to reproduce your issue)
  Initialised a repo and added a file or cloned a repo inside an existing
repo (think dependencies).
  See https://github.com/gitpod-io/gitpod/issues/5675 for background.

  In an existing repo:
    $ mkdir sub_test && cd sub_test/ && git init . && touch test.txt && git
add test.txt
    OR
    $ git clone https://github.com/stencila/test.git sub_test
    THEN
    $ git stash push -u

What did you expect to happen? (Expected behavior)
  Command should complete without error but ignore the directory (this is
the existing behavior prior to v2.31)
    $ git stash push -u
    Ignoring path sub_test
    Saved working directory and index state WIP on (no branch): 94f6e3e283
Git 2.30.2

What happened instead? (Actual behavior)
  Command failed
    $ git stash push -u
    error: sub_test/: is a directory - add files inside instead
    fatal: Unable to process path sub_test/
    Cannot save the untracked files

What's different between what you expected and what actually happened?
  Command failed

Anything else you want to add:
  It happens on all versions from v2.31 to current master.
  It is specifically related to this change:

https://github.com/git/git/commit/6e773527b6b03976cefbb0f9571bd40dd5995e6c#diff-70525b6b89c7cac91e520085d954a68671038d218b77d22855e938ab075a68d8L1006

  If this is the new expected behavior perhaps it can result in a better
error message and related documentation?

Please review the rest of the bug report below. You can delete any lines you don't wish to share.

[System Info] git version: git version 2.33.0.610.gcefe983a32 cpu: x86_64 built from commit: cefe983a320c03d7843ac78e73bd513a27806845 sizeof-long: 8 sizeof-size_t: 8 shell-path: /bin/sh uname: Linux 5.4.0-1051-gke #54-Ubuntu SMP Thu Aug 5 18:52:13 UTC 2021 x86_64 compiler info: gnuc: 9.3 libc info: glibc: 2.31 $SHELL (typically, interactive shell): /bin/bash

[Enabled Hooks]