GitPython currently infers the active branch from ref files directly, which breaks on repositories using Git’s reftable ref storage and can surface refs/heads/.invalid as the active branch. This PR switches active-branch resolution to Git’s symbolic-ref plumbing so behavior stays correct across ref backends.

• Active branch resolution

  • Replaced Repo.active_branch’s direct HEAD reference dereference path with git symbolic-ref -q HEAD.
  • Constructs Head from the resolved symbolic ref path returned by Git.
  • Preserves detached-HEAD semantics by translating symbolic-ref’s non-symbolic result into the existing TypeError behavior.

• Regression coverage

  • Added a reftable-specific test for empty repositories that validates:
    • repo.active_branch.path matches git symbolic-ref HEAD
    • repo.active_branch.name matches git symbolic-ref --short HEAD
  • Added a detached-HEAD test asserting Repo.active_branch raises TypeError with the expected detached-reference message.

@property
def active_branch(self) -> Head:
    try:
        return Head(self, self.git.symbolic_ref("-q", "HEAD").strip())
    except GitCommandError as err:
        if err.status == 1:
            raise TypeError("HEAD is a detached symbolic reference as it points to a commit") from err
        raise

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