bug: `git diff` implies folder isn’t a git repo

[allejo <me@allejo.io>]

Please answer the following questions to help us understand your issue.

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

Change ownership of a Git repo to require `safe.directory` behavior.

What did you expect to happen? (Expected behavior)

If you run `git diff --exit-code` on a non-`safe.directory`, you should get the standard error.

    Error: fatal: unsafe repository ('/path/to/repo' is owned by someone else)

What happened instead? (Actual behavior)

I get an error message saying it's not actually a git repository even though it is, it's just not safe to work in.

    warning: Not a git repository. Use --no-index to compare two paths outside a working tree
    usage: git diff --no-index [<options>] <path> <path>

    Diff output format options
        -p, --patch           generate patch
        -s, --no-patch        suppress diff output
        -u                    generate patch
        -U, --unified[=<n>]   generate diffs with <n> lines context
        -W, --function-context
                              generate diffs with <n> lines context
        --raw                 generate the diff in raw format
        --patch-with-raw      synonym for '-p --raw'
        --patch-with-stat     synonym for '-p --stat'
        --numstat             machine friendly --stat
        --shortstat           output only the last line of --stat
        -X, --dirstat[=<param1,param2>...]
                              output the distribution of relative amount of changes for each sub-directory
    ...

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

One error message implies the folder isn't a Git repo while the other explicitly says it's not safe to work in and how to fix it.

Anything else you want to add:

I am installing git on Ubuntu 20.04.4 from its default repositories inside of a GitHub Actions runner.

[System Info]
git version:
git version 2.25.1
cpu: x86_64
no commit associated with this build
sizeof-long: 8
sizeof-size_t: 8
shell-path: /bin/sh
libc info: no libc information available
$SHELL (typically, interactive shell): /bin/bash


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