Bug in "git diff" exit code with submodule and `--submodule=log`

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

Create a git repository that has a submodule. The git repository itself should be clean but the submodule should have changes that have been staged in the main repository. Then run: `git diff --exit-code --cached --submodule=log --; echo $?`

This shell script will reproduce the bug:

```shell #! /bin/sh

set -x

(mkdir sm; cd sm; git init .; git commit --allow-empty -m 'initial') mkdir test cd test git init . git commit --allow-empty -m 'initial' git submodule init git -c protocol.file.allow=always submodule add ../sm git add .gitmodules sm git commit -m 'add submodule sm' (cd sm; echo "hello" >greeting; git add greeting; git commit -m 'add greeting') git add sm # This exit code is correct: git diff --quiet --cached --submodule=short --; echo $? # This exit code is wrong: git diff --quiet --cached --submodule=log --; echo $? ```

What did you expect to happen? (Expected behavior)

The output of this "git diff" command should be `1`: ``` git diff --quiet --cached --submodule=log --; echo $? ```

What happened instead? (Actual behavior)

The output is `0`. The output is correct if the `--submodule=log` option is omitted.

``` % git diff --quiet --cached --submodule=log --; echo $? 0 % git diff --quiet --cached --submodule=short --; echo $? 1 ```

Anything else you want to add:

I believe this bug was introduced in git 2.46.0 or shortly before.

[System Info] git version: git version 2.46.0 cpu: x86_64 no commit associated with this build sizeof-long: 8 sizeof-size_t: 8 shell-path: /bin/sh feature: fsmonitor--daemon libcurl: 8.4.0 zlib: 1.2.12 uname: Darwin 23.6.0 Darwin Kernel Version 23.6.0: Mon Jul 29 21:13:00 PDT 2024; root:xnu-10063.141.2~1/RELEASE_X86_64 x86_64 compiler info: clang: 15.0.0 (clang-1500.3.9.4) libc info: no libc information available $SHELL (typically, interactive shell): /bin/zsh

[Enabled Hooks]