[BUG?] Weird interaction between `git -C`, aliases and worktrees

Hello all,

Up to recently I had an alias 'st' for `git status`, i.e. `git config alias.st status`. This works well in the main working tree as well as secondary worktrees (`git worktree add ...`), and also shows paths relative to the current directory if I'm not at the root of the repo, just like `git status`.

Now I wanted to change my alias so that it ran `git status` and then an additional command, i.e. `git config alias.st '!git status && date'` (for demonstration purposes). This works correctly in the main worktree and secondary worktrees, but since aliases run from the root of the repo, the paths in the output of 'git st' are not relative to the current directory.

So my next attempt was `git config alias.st '!git -C "${GIT_PREFIX}" status && date'`, so that paths would be relative in the output. However, this works in the main worktree but fails badly in the secondary worktree:

``` rm -rf test && rm -rf test2 && git init test && cd test && mkdir folder && date>>folder/file && git add folder/file && git ci -m "commit" && git config alias.st '!git -C "${GIT_PREFIX}" status && date' && date >>folder/file && echo '=== `git st` in main worktree at root ===' && git st && cd folder && echo '=== `git st` in main worktree in folder ===' && git st && git worktree add ../test2 && cd ../test2 && date >>folder/file && echo '=== `git st` in secondary worktree at root ===' && git st && cd folder && echo '=== `git st` in secondary worktree in folder ===' && git st ```

The last commands ouputs: 

```
=== `git st` in secondary worktree in folder ===
On branch test2
Changes not staged for commit:
  (use "git add/rm <file>..." to update what will be committed)
  (use "git restore <file>..." to discard changes in working directory)
	deleted:    folder/file

Untracked files:
  (use "git add <file>..." to include in what will be committed)
	file

no changes added to commit (use "git add" and/or "git commit -a") ```

So something is wrong in the automatic worktree path detection... If I replace the alias with

    git config alias.st '!git -C "${GIT_PREFIX}" --work-tree=$PWD status && date' 

then it works correctly, but I have a feeling this should not be necessary...

I've CC-ed Eric (because of his work on `git worktree`) and Elijah (because I remember he worked on `dir.c` so maybe this is related to that code, but I'm not sure).

Cheers, Philippe