Opening git-bash from git-gui and cd'ing to the root of the filesystem results in git-bash thinking it is at the top-level of the repo

Hi,

Generated this bug report using git-bugreport:

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)

    Note: Was redirected from
[here](https://github.com/git-for-windows/git/issues/5234), on the
speculation that the bug report might not be Windows-specific.

    1. Open git-gui from the Windows start menu (or whatever the
equivalent on Linux is)
    2. Select an existing repo, either through `Open Existing
Repository` or one of the paths below `Open Recent Repository`
    3. From the top menu, click `Repository`, click `Git Bash`
    4. In git-bash, `cd /` (expecting `/` to *not* to be a git repo),
then `git status`

What did you expect to happen? (Expected behavior)

    fatal: not a git repository (or any of the parent directories): .git

What happened instead? (Actual behavior)

    On branch foo
    Your branch is up to date with 'origin/foo'.

    nothing to commit, working tree clean

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

    The output messages. The expected behavior is to fatal. The actual
behavior did *not* fatal. In both cases, the current working directory
is at the filesystem root, which is expected to not be a git repo.

Anything else you want to add:

    It seems that what is affecting this is that opening up git-gui
sets the `GIT_DIR` env var automatically. The resulting behavior seems
somewhat non-intuitive and confused myself and a colleague. The
resulting behavior I expected was the behavior one would get if
git-gui spawned a git-bash shell with the current working directory
set to the repo's path, but *without* setting `GIT_DIR`.

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 2.45.0.windows.1
    cpu: x86_64
    built from commit: b5d0511969ccd9ab86395c37e5a7619d8b4e7c32
    sizeof-long: 4
    sizeof-size_t: 8
    shell-path: /bin/sh
    feature: fsmonitor--daemon
    uname: Windows 10.0 19045
    compiler info: gnuc: 13.2
    libc info: no libc information available
    $SHELL (typically, interactive shell): C:\Program Files\Git\usr\bin\bash.exe

    [Enabled Hooks]
    post-checkout
    post-commit
    post-merge
    pre-push

- Kevin Navero