Bug with worktrees...

My apologies if this has already been reported, but I ran into an interesting bug with worktrees. In particular, I have an alias 'st' that maps to 'status -sb'. When running this under a subdirectory of a worktree created with 'git worktree add', it fails complaining that the work tree has already been set.

Here's a script to reproduce the problem:
    git init test-repo
    cd test-repo
    git config --local alias.st 'status -sb'
    mkdir subdir
    echo file > subdir/file.txt
    git add subdir/file.txt
    git commit -m 'add file'
    git branch foo
    git worktree add ../new-worktree foo
    cd ../new-worktree/subdir
    echo "new line" >> file.txt
    echo "this will work"
    git status -sb
    echo "this fails"
    git st

When I run it, I see this:

Initialized empty Git repository in
/home/jszakmeister/tmp/test-case/test-repo/.git/
[master (root-commit) 1ec5360] add file
 1 file changed, 1 insertion(+)
 create mode 100644 subdir/file.txt
Enter ../new-worktree (identifier new-worktree)
Switched to branch 'foo'
this will work
## foo
this fails
fatal: internal error: work tree has already been set
Current worktree: /home/jszakmeister/tmp/test-case/new-worktree
New worktree: /home/jszakmeister/tmp/test-case/new-worktree/subdir

Hope this helps!

-John