fix: resolve active_branch correctly for reftable refs

Co-authored-by: Byron <63622+Byron@users.noreply.github.com>
Agent-Logs-Url: https://github.com/gitpython-developers/GitPython/sessions/cb73f600-a42a-4400-b68c-da530d57677f

Author: Copilot

git/repo/base.py

@@ -1045,8 +1045,12 @@ def active_branch(self) -> Head:
         :return:
             :class:`~git.refs.head.Head` to the active branch
         """
-        # reveal_type(self.head.reference)  # => Reference
-        return self.head.reference
+        try:
+            return Head(self, self.git.symbolic_ref("HEAD").strip())
+        except GitCommandError as err:
+            if err.status == 1 or "not a symbolic ref" in str(err).lower():
+                raise TypeError("HEAD is a detached symbolic reference as it points to a commit") from err
+            raise
 
     def blame_incremental(self, rev: str | HEAD | None, file: str, **kwargs: Any) -> Iterator["BlameEntry"]:
         """Iterator for blame information for the given file at the given revision.

test/test_repo.py

@@ -962,6 +962,20 @@ def test_empty_repo(self, rw_dir):
 
         assert "BAD MESSAGE" not in contents, "log is corrupt"
 
+    @with_rw_directory
+    def test_empty_repo_reftable_active_branch(self, rw_dir):
+        git = Git(rw_dir)
+        try:
+            git.init(ref_format="reftable")
+        except GitCommandError as err:
+            if err.status == 129:
+                pytest.skip("git init --ref-format is not supported by this git version")
+            raise
+
+        repo = Repo(rw_dir)
+        self.assertEqual(repo.active_branch.name, "master")
+        assert not repo.active_branch.is_valid(), "Branch is yet to be born"
+
     def test_merge_base(self):
         repo = self.rorepo
         c1 = "f6aa8d1"