Address review feedback about dangling commondir

Review feedback noted that a dangling commondir symlink was treated as absent, allowing local objects and refs to validate the repository.

Distinguish a truly missing commondir from a dangling symlink. This follows Git's get_common_dir_noenv(), whose file_exists check uses lstat before attempting to read the entry.

Git baseline: 15c6308cf7ad276b306aa5b3ababfbdebfb1a917, setup.c get_common_dir_noenv() and dir.c file_exists().

Validation: 8 focused tests and 12 subtests; Ruff check and format; mypy; compileall; git diff --check.

Author: codex

git/repo/fun.py

@@ -87,9 +87,12 @@ def is_git_dir(d: PathLike) -> bool:
         if common_dir == "":
             return False
         if common_dir is None:
+            common_dir_file = Path(d) / "commondir"
             try:
-                common_dir = os.fsdecode((Path(d) / "commondir").read_bytes()).rstrip("\r\n")
+                common_dir = os.fsdecode(common_dir_file.read_bytes()).rstrip("\r\n")
             except FileNotFoundError:
+                if osp.lexists(common_dir_file):
+                    return False
                 common_dir = os.fspath(d)
             except OSError:
                 return False

test/test_repo.py

@@ -181,6 +181,17 @@ def test_repo_discovery_rejects_invalid_metadata(self):
                 with self.subTest(metadata=".git", contents=contents):
                     self.assertRaises(InvalidGitRepositoryError, Repo, path)
 
+    @requires_symlinks
+    def test_repo_discovery_rejects_dangling_commondir(self):
+        with tempfile.TemporaryDirectory() as tdir:
+            path = Path(tdir)
+            (path / "objects").mkdir()
+            (path / "refs").mkdir()
+            (path / "HEAD").write_text("ref: refs/heads/main\n")
+            (path / "commondir").symlink_to("missing")
+
+            self.assertRaises(InvalidGitRepositoryError, Repo, path)
+
     def test_repo_discovery_uses_storage_environment(self):
         with tempfile.TemporaryDirectory() as tdir:
             git_dir = Path(tdir) / "git"