adding unstaged changes when not in "work tree" yields only ambiguous error

**Describe the bug**
When staging changes:
```
staging all error:
git error:invalid path: 'my-path-here'; class=Index (10)
```

A git repository is created using the `--git-dir=/path/to/$FOLDER` which will create the standard `.git` contents separate from the working files. If inside this folder gitui will not complain and will incorrectly show unstaged changes and not give suitable errors.

**To Reproduce**
``` sh
mkdir -p ~/working_tree ~/git_folder && cd ~/working_tree
git init --git-dir ~/git_folder
touch readme.txt && git commit -a -c "init"
cd ~/.git_folder
git status
# fatal: this operation must be run in a work tree
# gitui > "a" to add all files > error as specified at the top
```
**Expected behavior**
A clear and concise error similar to regular git commands.

**Context (please complete the following information):**
- OS/Distro + Version: `Arch / EndeavourOS`
- GitUI Version: `nightly 2024-04-15`
- Rust version: `1.27.0`

**Additional context**
I expect many more commands to fail for similar reasons. No others tested. The porcelain git commands give a more operable error.

Reproduce the issue with a repository created using a separate --git-dir, then inspect the code handling the "a" action and the resulting Git errors. Confirm the behavior against git status and staging commands; done means repositories without a work tree produce a clear, operable error instead of the ambiguous invalid-path message.

Scritto dal modello di indicizzazione a partire dal testo della issue.