GitUI shows working-tree changes in cross-linked bare repos where Git reports no work tree

[Yi2255]

Describe the bug
In a mutual bare-repo reference setup, gitui shows working-tree file changes while Git CLI reports there is no work tree.

Example mismatch:

• git status in /i4/repo0 returns: fatal: this operation must be run in a work tree
• gitui in /i4/repo0 still shows many unstaged entries (HEAD, config, hooks/, etc.)

This is misleading because the repo is not in a valid work-tree state for normal file status operations.

To Reproduce
Steps to reproduce the behavior:

1. Create two repos at fixed reproducible paths:

   mkdir -p /i4/repo0 /i4/repo1

2. Initialize and cross-link git directories:

   cd /i4/repo0
   git init --bare
   git init --separate-git-dir=../repo1
   
   cd /i4/repo1
   git init --bare
   git init --separate-git-dir=../repo0

3. Verify CLI behavior in /i4/repo0:

   cd /i4/repo0
   git status

   Expected CLI output:

   fatal: this operation must be run in a work tree
   

4. Run gitui in the same path:

   cd /i4/repo0
   gitui

5. Observe that gitui still lists unstaged/untracked files.When Git reports no work tree, gitui should not show normal working-tree file changes. It should present an error/state consistent with Git CLI.