Bug with git worktrees and git init

[Max Nordlund <max.nordlund@sqore.com>]

Hi,

I've been using multiple worktrees for months without issue (it's a
great feature, thanks), until recently when I wanted to add hooks to
them. So, when I added a template for the hooks, everything was fine
until I did a git reset --hard in the original repo which both applied
those changes to the other worktrees' working tree (the files on
disk), and made my master branch kinda lose it's connection to the
remote/think it was a kinda bare repo.

To reproduce this:

```
mkdir source-repo
cd source-repo
git init
touch foo
git add foo
git commit -m 'Add foo'
git worktree add ../worktree # which also creates a new branch 'worktree'
touch bar
git add bar
git commit -m 'Add bar'
cd ../worktree
git init
cd ../source-repo
git reset --hard master
cd ../worktree
git status # Suddenly `bar` has appear the working tree and is not tracked
```

I don't really now what is up with this, but I did notice that it is
the last worktree in which git init has been run that is affected. I
only ran git init to copy the hooks from the template, but if that is
not something you should do in a worktree then a check would have been
nice.

Thanks for this awesome tool, and I hope this helps
Max Nordlund