Use more ligtweight approach to guarantee deletion

This changes from `@with_rw_directory` back to TemporaryDirectory,
but calls git.util.rmtree on the repsitory's .git directory where
some read-only files otherwise cause TemporaryDirectory's cleanup
to raise PermissionError on Windows in Python 3.7.

This avoids the gc.collect that is known not to be necessary in
this speciifc situation, as well as the problem that, if operating
in the temporary directory did fail, then name of the helper would
be logged as the name of the test where the failure occurred. But
this has the disadvantage of making the helper more complex and
harder to understand. So this may not be the best approach either.

test/test_tree.py

@@ -7,10 +7,11 @@
 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, with_rw_directory
+from git.util import cwd, rmtree
+from test.lib import TestBase
 
 
 class TestTree(TestBase):
@@ -42,35 +43,39 @@ def test_serializable(self):
             testtree._deserialize(stream)
         # END for each item in tree
 
-    @with_rw_directory
-    def _get_git_ordered_files(self, rw_dir):
+    @staticmethod
+    def _get_git_ordered_files():
         """Get files as git orders them, to compare in test_tree_modifier_ordering."""
-        # 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]
+        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()
+
+            try:
+                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()
+            finally:
+                rmtree(Path(tdir, ".git"))
+
+            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."""