Passage does not recognize git repository in `.passage` dir

Hello,

Many thanks for your work on `age` and `passage`. I recently switched to both from `gpg` and `pass`. Key management is much easier with age! :)

Instead of putting `.passage/store/` into a git repository, I track the whole `.passage` dir (identities are stored on a yubikey). The git subcommand does not recognize that the passage dir is a repository.

I narrowed it down to line 33 in password-store.sh:

```
[[ $(git -C "$INNER_GIT_DIR" rev-parse --is-inside-work-tree 2>/dev/null) == true ]] || INNER_GIT_DIR=""
```

This test somehow fails (and resets the variable to `""`) even though executing this test manually results in the correct `true` result. I do not understand why this test does not detect correctly that the `.passage` dir is a git repository in the script but executing it manually gives the correct result.

The `GIT_CEILING_DIRECTORIES` seems to be correctly set to the home directory.

Can somebody help me out here?

Thanks for your help!