[bug report] git diff --relative not doing well with worktree in hooks

[Antoine Bolvy <antoine.bolvy@gmail.com>]

Hello,

I noticed a weird behavior when using git diff --relative with worktrees and
hooks. When called from a pre-commit hook from a worktree, the relative option
has no effect.

Here is how to reproduce the issue:

```bash
mkdir hook-repro && cd hook-repro
git init test && cd test
mkdir folder && touch folder/.gitkeep && git add folder
git commit -m 'init'
cat <<EOF > .git/hooks/pre-commit
#!/bin/bash

cd folder || exit

pwd # display the current working directory

git diff --cached --relative --name-only
EOF
chmod +x .git/hooks/pre-commit
```

```bash
echo "foo" > folder/bar
git add folder
git commit -m "test"
```

Displays
```
/home/arch/git/awfus/hook-repro/test/folder
bar
```

Now creating a worktree:

```bash
git worktree add ../worktree && cd ../worktree
echo "bar" > folder/foo
git add folder
git commit -m "worktree"
```

Displays
```
/home/arch/git/awfus/hook-repro/worktree/folder
folder/foo
```

The path is no longer show relative. This causes issues with more complex
scripts.

Git version: 2.45.0 (x86_64) on Arch Linux, shell is zsh (bash for the hook
script)

Let me know if you need any more information :)