Fix new PermissionError in Windows with Python 3.7

In Python 3.7, tempfile.TemporaryDirectory doesn't delete files on
cleanup whose permissions have to be changed to be deleted.

This uses `@with_rw_directory` instead, though that may be overkill
here.

test/test_tree.py

@@ -7,11 +7,10 @@
 import os.path as osp
 from pathlib import Path
 import subprocess
-import tempfile
 
 from git.objects import Tree, Blob
 from git.util import cwd
-from test.lib import TestBase
+from test.lib import TestBase, with_rw_directory
 
 
 class TestTree(TestBase):
@@ -43,36 +42,35 @@ def test_serializable(self):
             testtree._deserialize(stream)
         # END for each item in tree
 
-    @staticmethod
-    def _get_git_ordered_files():
+    @with_rw_directory
+    def _get_git_ordered_files(self, rw_dir):
         """Get files as git orders them, to compare in test_tree_modifier_ordering."""
-        with tempfile.TemporaryDirectory() as tdir:
-            # Create directory contents.
-            Path(tdir, "file").mkdir()
-            for filename in (
-                "bin",
-                "bin.d",
-                "file.to",
-                "file.toml",
-                "file.toml.bin",
-                "file0",
-            ):
-                Path(tdir, filename).touch()
-            Path(tdir, "file", "a").touch()
-
-            with cwd(tdir):
-                # Prepare the repository.
-                subprocess.run(["git", "init", "-q"], check=True)
-                subprocess.run(["git", "add", "."], check=True)
-                subprocess.run(["git", "commit", "-m", "c1"], check=True)
-
-                # Get git output from which an ordered file list can be parsed.
-                rev_parse_command = ["git", "rev-parse", "HEAD^{tree}"]
-                tree_hash = subprocess.check_output(rev_parse_command).decode().strip()
-                cat_file_command = ["git", "cat-file", "-p", tree_hash]
-                cat_file_output = subprocess.check_output(cat_file_command).decode()
-
-            return [line.split()[-1] for line in cat_file_output.split("\n") if line]
+        # Create directory contents.
+        Path(rw_dir, "file").mkdir()
+        for filename in (
+            "bin",
+            "bin.d",
+            "file.to",
+            "file.toml",
+            "file.toml.bin",
+            "file0",
+        ):
+            Path(rw_dir, filename).touch()
+        Path(rw_dir, "file", "a").touch()
+
+        with cwd(rw_dir):
+            # Prepare the repository.
+            subprocess.run(["git", "init", "-q"], check=True)
+            subprocess.run(["git", "add", "."], check=True)
+            subprocess.run(["git", "commit", "-m", "c1"], check=True)
+
+            # Get git output from which an ordered file list can be parsed.
+            rev_parse_command = ["git", "rev-parse", "HEAD^{tree}"]
+            tree_hash = subprocess.check_output(rev_parse_command).decode().strip()
+            cat_file_command = ["git", "cat-file", "-p", tree_hash]
+            cat_file_output = subprocess.check_output(cat_file_command).decode()
+
+        return [line.split()[-1] for line in cat_file_output.split("\n") if line]
 
     def test_tree_modifier_ordering(self):
         """TreeModifier.set_done() sorts files in the same order git does."""