[BUG] `git gc` or `git pack-refs` wipes all notes for `git notes` command

Steps to reproduce:
1. Create some notes with the `git notes add` command.
2. Run `git notes` to list stored notes.
3. Run any of `git gc  --prune=now` or `git pack-refs --all` commands.
4. Run `git notes` again and see that there are no more seen notes.

The reason is that all commands like `git notes` or `git log --notes` expect to find notes head as unpacked ref in .git/refs/notes/commits. But the gc or the pack-refs command packs .git/refs/notes/commits ref into .git/packed-refs file.

It's possible to restore the notes ref with a shell script like:
```
fix-notes-unpack-commits-ref(){
    local hash path
    grep refs/notes/commits .git/packed-refs |\
    while read hash path; do
        path=".git/$path"
        mkdir -p "$(dirname "$path")"
        echo -n "$hash" > "$path"
    done
}
```
Or, fortunately, from another backup place: .git/logs/refs/notes/commits.

The expected behavior is that `git notes` and `git log` should work also with notes ref stored in the .git/packed-refs file and also respect that the `git notes` command can store notes with a custom ref by using the `--ref` option.

Yours sincerely, Andrew