Regression in `git diff --quiet HEAD` when a new file is staged

[Jake Zimmerman <jake@zimmerman.io>]

In git v2.51.1, `git diff --quiet HEAD` will actually print something
if the diff output includes a new, staged file.

## To reproduce

    ❯ mkdir foo
    ❯ cd foo
    ❯ git init .
    Initialized empty Git repository in /Users/jez/foo/.git/
    ❯ gc --allow-empty -m "Initial empty commit"
    [master (root-commit) 858966f] Initial empty commit
    ❯ touch foo.txt
    ❯ git add foo.txt
    ❯ git diff --quiet HEAD

On git v2.51.0, the output of the last command is empty.
On git v2.51.1, the output of the last command is this:

    diff --git a/foo.txt b/foo.txt
    new file mode 100644
    index 0000000..e69de29

## Expected behavior

The stated docs for `--quiet`: "Disable all output of the program," so
I expect there to be no output, like in older versions.

## Likely cause

I ran a git bisect and isolated this commit:

b55e6d36ebce69136559add8fffd1a65df231518
( https://github.com/git/git/commit/e1d3d61a45bfdc5031d2066c0e4505ebd8145777 )

"diff: ensure consistent diff behavior with ignore options"