Add a unit test for CVE-2023-40590

This adds test_it_executes_git_not_from_cwd to verify that the
execute method does not use "git.exe" in the current directory on
Windows, nor "git" in the current directory on Unix-like systems,
when those files are executable.

It adds a _chdir helper context manager to support this, because
contextlib.chdir is only available on Python 3.11 and later.

test/test_git.py

@@ -4,10 +4,12 @@
 #
 # This module is part of GitPython and is released under
 # the BSD License: http://www.opensource.org/licenses/bsd-license.php
+import contextlib
 import os
+import shutil
 import subprocess
 import sys
-from tempfile import TemporaryFile
+from tempfile import TemporaryDirectory, TemporaryFile
 from unittest import mock
 
 from git import Git, refresh, GitCommandError, GitCommandNotFound, Repo, cmd
@@ -20,6 +22,17 @@
 from git.compat import is_win
 
 
+@contextlib.contextmanager
+def _chdir(new_dir):
+    """Context manager to temporarily change directory. Not reentrant."""
+    old_dir = os.getcwd()
+    os.chdir(new_dir)
+    try:
+        yield
+    finally:
+        os.chdir(old_dir)
+
+
 class TestGit(TestBase):
     @classmethod
     def setUpClass(cls):
@@ -75,6 +88,23 @@ def test_it_transforms_kwargs_into_git_command_arguments(self):
     def test_it_executes_git_to_shell_and_returns_result(self):
         self.assertRegex(self.git.execute(["git", "version"]), r"^git version [\d\.]{2}.*$")
 
+    def test_it_executes_git_not_from_cwd(self):
+        with TemporaryDirectory() as tmpdir:
+            if is_win:
+                # Copy an actual binary executable that is not git.
+                other_exe_path = os.path.join(os.getenv("WINDIR"), "system32", "hostname.exe")
+                impostor_path = os.path.join(tmpdir, "git.exe")
+                shutil.copy(other_exe_path, impostor_path)
+            else:
+                # Create a shell script that doesn't do anything.
+                impostor_path = os.path.join(tmpdir, "git")
+                with open(impostor_path, mode="w", encoding="utf-8") as file:
+                    print("#!/bin/sh", file=file)
+                os.chmod(impostor_path, 0o755)
+
+            with _chdir(tmpdir):
+                self.assertRegex(self.git.execute(["git", "version"]), r"^git version [\d\.]{2}.*$")
+
     def test_it_accepts_stdin(self):
         filename = fixture_path("cat_file_blob")
         with open(filename, "r") as fh: