bugreport: "git config --global" fails if an invalid .git file present

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

Executing `git config --global ...` and $PWD/.git exists as a file and is invalid.

What did you expect to happen? (Expected behavior)

Git reads from or writes to the global config store.

What happened instead? (Actual behavior)

It prints:
  fatal: invalid gitfile format: $PWD/.git

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

Git fails to access the global config even if the current directory's `.git` file has nothing to do with the global config.

[System Info] git version: git version 2.37.2 cpu: arm64 no commit associated with this build sizeof-long: 8 sizeof-size_t: 8 shell-path: /bin/sh feature: fsmonitor--daemon uname: Darwin 21.6.0 Darwin Kernel Version 21.6.0: Sat Jun 18 17:07:22 PDT 2022; root:xnu-8020.140.41~1/RELEASE_ARM64_T6000 arm64 compiler info: clang: 13.1.6 (clang-1316.0.21.2.5) libc info: no libc information available $SHELL (typically, interactive shell): /opt/homebrew/bin/bash

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