Bug report: Strange behavior with `git gc` and `reference-transaction` hook

Hi all,

I'm seeing unusual behavior on Git built from source at cd3e606211bb1cf8bc57f7d76bab98cc17a150bc, but which appears to extend back to Git v2.29.

This is a repro script:

``` #!/bin/sh

temp_dir=$(mktemp -d) echo "git dir is: $temp_dir" mkdir -p "$temp_dir" trap "rm -rf $temp_dir" EXIT

cd "$temp_dir" || exit 1 git init -q date='Thu, 07 Apr 2005 22:13:13 +0200' GIT_AUTHOR_DATE="$date" GIT_COMMITTER_DATE="$date" git commit -q --allow-empty -m 'Initial commit'

mkdir -p '.git/hooks' cat >.git/hooks/reference-transaction <<'EOT' #!/bin/sh

[[ "$1" != 'committed' ]] && exit 0 echo 'New reference-transaction invocation'

while read old new ref; do
  echo "  old: $old, new: $new, ref: $ref"
done
EOT
chmod +x .git/hooks/reference-transaction

git gc --prune=now -q git show-ref refs/heads/master ```

And this is the output it produces:

```
git dir is: /var/folders/gn/gdp9z_g968b9nx7c9lvgy8y00000gp/T/tmp.b3Jc6qnb
New reference-transaction invocation
  old: 0000000000000000000000000000000000000000, new:
e197d18c017d4038418be8b1cd38f4503e289165, ref: refs/heads/master
New reference-transaction invocation
  old: e197d18c017d4038418be8b1cd38f4503e289165, new:
0000000000000000000000000000000000000000, ref: refs/heads/master
e197d18c017d4038418be8b1cd38f4503e289165 refs/heads/master
```

These hooks are invoked a few milliseconds one after another.

The expected behavior would be that the latest reference transaction hook refers to the state of the references on disk. That is, either `master` should point to 0 (be deleted), or it should have said that `master` pointed to `e197d1`.

But if we actually examine `master`, it's set to `e197d1`, just as you would expect. The GC should have been a no-op overall.

Best, Waleed