Support git worktree in `nbdev_install_hooks`

When running `nbdev_install_hooks`, I get

```
NotADirectoryError: [Errno 20] Not a directory: '/dir1/.git/hooks'
```
This makes sense, because I'm using git's worktrees:

```
% cat .git
gitdir: /dir2/.git/worktrees/dir1
```

If I understand correctly, the issue is [here](https://github.com/fastai/nbdev/blob/master/nbdev/clean.py#L195-L199), where the code assumes that the `.git` directory is `$(git rev-parse --show-toplevel)/.git`; instead, it should ask `git` directly via `git rev-parse --git-dir`.