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

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