Accept UNC share roots as rooted paths

Author: Byron

git/util.py

@@ -324,9 +324,9 @@ def _is_path_rooted(path: PathLike) -> bool:
     On POSIX, which has no drive concept, this simply distinguishes absolute
     paths such as ``/directory`` from relative paths such as ``directory``.
     """
-    _drive, tail = osp.splitdrive(os.fspath(path))
+    drive, tail = osp.splitdrive(os.fspath(path))
     separators = (os.sep,) if os.altsep is None else (os.sep, os.altsep)
-    return tail.startswith(separators)
+    return tail.startswith(separators) or bool(drive) and osp.isabs(path)
 
 
 def _to_relative_path(root: PathLike, path: PathLike) -> str:

test/test_index.py

@@ -1090,6 +1090,15 @@ class Mocked:
         actual_path = index._to_relative_path(path)
         self.assertEqual(expected_path, actual_path)
 
+    @pytest.mark.skipif(sys.platform != "win32", reason="Specifically for Windows.")
+    def test__to_relative_path_windows_unc_share_root(self):
+        for repo_root in [R"\\server\share", R"\\?\UNC\server\share"]:
+            with self.subTest(repo_root=repo_root):
+                repo = mock.Mock(bare=False, git_dir=repo_root, working_tree_dir=repo_root)
+                index = IndexFile(repo)
+
+                self.assertEqual(index._to_relative_path(repo_root), ".")
+
     @pytest.mark.skipif(sys.platform != "win32", reason="Specifically for Windows.")
     @with_rw_directory
     def test__to_relative_path_windows_path_kinds(self, rw_dir):