`active_branch` is detected incorrectly when using `reftable`

alynn-coefficient

On git repositories using the new reftable format (slated to become the default in the upcoming Git 3.0.0), GitPython detects the wrong ref as being active.

Observe, with the traditional "files" format:

mkdir test
cd test
git init --ref-format=files
uvx --with gitpython python -c "import git; print(git.Repo('.').active_branch.name)"

Produces main as expected. However:

mkdir test
cd test
git init --ref-format=reftable
uvx --with gitpython python -c "import git; print(git.Repo('.').active_branch.name)"

Produces .invalid.