Fix CVE-2023-40590

This fixes the path search bug where the current directory is
included on Windows, by setting NoDefaultCurrentDirectoryInExePath
for the caller. (Setting for the callee env would not work.)

This sets it only on Windows, only for the duration of the Popen
call, and then automatically unsets it or restores its old value.

NoDefaultCurrentDirectoryInExePath is documented at:
https://learn.microsoft.com/en-us/windows/win32/api/processenv/nf-processenv-needcurrentdirectoryforexepatha

It automatically affects the behavior of subprocess.Popen on
Windows, due to the way Popen uses the Windows API. (In contrast,
it does not, at least currently on CPython, affect the behavior of
shutil.which. But shutil.which is not being used to find git.exe.)

git/cmd.py

@@ -5,7 +5,7 @@
 # the BSD License: http://www.opensource.org/licenses/bsd-license.php
 from __future__ import annotations
 import re
-from contextlib import contextmanager
+import contextlib
 import io
 import logging
 import os
@@ -14,6 +14,7 @@
 import subprocess
 import threading
 from textwrap import dedent
+import unittest.mock
 
 from git.compat import (
     defenc,
@@ -963,8 +964,11 @@ def execute(
                     redacted_command,
                     '"kill_after_timeout" feature is not supported on Windows.',
                 )
+            # Only search PATH, not CWD. This must be in the *caller* environment. The "1" can be any value.
+            patch_caller_env = unittest.mock.patch.dict(os.environ, {"NoDefaultCurrentDirectoryInExePath": "1"})
         else:
             cmd_not_found_exception = FileNotFoundError  # NOQA # exists, flake8 unknown @UndefinedVariable
+            patch_caller_env = contextlib.nullcontext()
         # end handle
 
         stdout_sink = PIPE if with_stdout else getattr(subprocess, "DEVNULL", None) or open(os.devnull, "wb")
@@ -980,21 +984,21 @@ def execute(
             istream_ok,
         )
         try:
-            proc = Popen(
-                command,
-                env=env,
-                cwd=cwd,
-                bufsize=-1,
-                stdin=istream or DEVNULL,
-                stderr=PIPE,
-                stdout=stdout_sink,
-                shell=shell is not None and shell or self.USE_SHELL,
-                close_fds=is_posix,  # unsupported on windows
-                universal_newlines=universal_newlines,
-                creationflags=PROC_CREATIONFLAGS,
-                **subprocess_kwargs,
-            )
-
+            with patch_caller_env:
+                proc = Popen(
+                    command,
+                    env=env,
+                    cwd=cwd,
+                    bufsize=-1,
+                    stdin=istream or DEVNULL,
+                    stderr=PIPE,
+                    stdout=stdout_sink,
+                    shell=shell is not None and shell or self.USE_SHELL,
+                    close_fds=is_posix,  # unsupported on windows
+                    universal_newlines=universal_newlines,
+                    creationflags=PROC_CREATIONFLAGS,
+                    **subprocess_kwargs,
+                )
         except cmd_not_found_exception as err:
             raise GitCommandNotFound(redacted_command, err) from err
         else:
@@ -1144,7 +1148,7 @@ def update_environment(self, **kwargs: Any) -> Dict[str, Union[str, None]]:
                 del self._environment[key]
         return old_env
 
-    @contextmanager
+    @contextlib.contextmanager
     def custom_environment(self, **kwargs: Any) -> Iterator[None]:
         """
         A context manager around the above ``update_environment`` method to restore the