"git diff-files" command reports differences that don't exist

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

1. Be in any existing repository in a clean state (I tested with 2 existing
   repositories and 1 completely fresh)
1. run `git diff-files`, see no output
1. touch an existing file: `touch a`
1. run `git diff-files`

What did you expect to happen? (Expected behavior)

`git diff-files` should show no output

What happened instead? (Actual behavior)

`git diff-files` reported a difference on the touched file.

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

git reports a difference where none exists

Anything else you want to add:

This difference vanished after running other commands (e.g. `git diff`).

A complete reproduction using docker is below:

$ docker run -it --rm debian:latest
root@d24ec0793bdf:/# apt update && apt -y install git
…
root@d24ec0793bdf:/# mkdir ~/new-repo
root@d24ec0793bdf:/# cd ~/new-repo
root@d24ec0793bdf:~/new-repo# git init
Initialized empty Git repository in /root/new-repo/.git/
root@d24ec0793bdf:~/new-repo# touch a
root@d24ec0793bdf:~/new-repo# git add a
root@d24ec0793bdf:~/new-repo# git config --global user.email
'user@example.net'
root@d24ec0793bdf:~/new-repo# git config --global user.name 'Example User'
root@d24ec0793bdf:~/new-repo# git commit -m 'initial'
[master (root-commit) 791011f] initial
 1 file changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 a
root@d24ec0793bdf:~/new-repo# git diff-files
root@d24ec0793bdf:~/new-repo# touch a
root@d24ec0793bdf:~/new-repo# git diff-files
:100644 100644 e69de29bb2d1d6434b8b29ae775ad8c2e48c5391
0000000000000000000000000000000000000000 M    a
root@d24ec0793bdf:~/new-repo# git diff
root@d24ec0793bdf:~/new-repo# git diff-files
root@d24ec0793bdf:~/new-repo# git --version
git version 2.20.1

[System Info] git version: git version 2.30.1 cpu: x86_64 no commit associated with this build sizeof-long: 8 sizeof-size_t: 8 shell-path: /bin/sh uname: Linux 5.11.1-arch1-1 #1 SMP PREEMPT Tue, 23 Feb 2021 14:05:30 +0000 x86_64 compiler info: gnuc: 10.2 libc info: glibc: 2.33 $SHELL (typically, interactive shell): /bin/bash

This same behaviour was also found in:

* 2.20.1
* 2.25.1

[Enabled Hooks] (none)