Bug: All git operations fail when .git contains a non-existent gitdir

[Tom Cook <tom.k.cook@gmail.com>]

What did you do before the bug happened? (Steps to reproduce your issue)

Add a git submodule to a git repository.
Overlay-mount that submodule to another place in the filesystem.
Attempt any git operation in the overlay-mounted path.

What did you expect to happen? (Expected behaviour)

git operations that don't depend on a repository should succeed.

What happened instead? (Actual behaviour)

All git operations fail.  Even `git bugreport` fails.  More importantly,
`git ls-remote` on an unrelated repository fails.

```
$ git bugreport
fatal: not a git repository:
/home/tkcook/git/Veea/demo.git/VHP-3375-veeadb-on-vhe09/build/iesv10/programs/veeadb/../../../../../../.git/worktree>
```

What's different between what you expected and what actually happened?

Commands that should succeed actually fail with the above message.

Anything else you want to add:

Please review the rest of the bug report below.
You can delete any lines you don't wish to share.


[System Info]
git version:
git version 2.30.2
cpu: x86_64
no commit associated with this build
sizeof-long: 8
sizeof-size_t: 8
shell-path: /bin/sh
uname: Linux 5.13.0 #1 SMP Mon Jun 28 11:38:29 UTC 2021 x86_64
compiler info: gnuc: 10.2
libc info: glibc: 2.33
$SHELL (typically, interactive shell): /bin/bash


[Enabled Hooks]
not run from a git repository - no hooks to show