Further expand "not from cwd" test re: cmd.exe

On Python versions for which python/cpython#101283 is not patched,
using Popen with shell=True can find cmd.exe in the current
directory on Windows, in the rare case that the ComSpec environment
variable is not defined.

This is not necessarily worth addressing, because it is a bug in
CPython rahter than GitPython, because that bug has been patched,
and because it is very rare that ComSpec is undefined. However:

- Changing the code to avoid it would also make that code simpler.
- Patched versions of Python <=3.9 don't have python.org builds.

This commit just expands the test to add cases where a repository
also has a file cmd.exe and where ComSpec is undefined, showing
that this case is not covered.

test/test_git.py

@@ -29,6 +29,21 @@
 from test.lib import TestBase, fixture_path, with_rw_directory
 
 
+@contextlib.contextmanager
+def _patch_out_env(name):
+    try:
+        old_value = os.environ[name]
+    except KeyError:
+        old_value = None
+    else:
+        del os.environ[name]
+    try:
+        yield
+    finally:
+        if old_value is not None:
+            os.environ[name] = old_value
+
+
 @ddt.ddt
 class TestGit(TestBase):
     @classmethod
@@ -137,14 +152,17 @@ def test_it_executes_git_and_returns_result(self):
         self.assertRegex(self.git.execute(["git", "version"]), r"^git version [\d\.]{2}.*$")
 
     @ddt.data(
-        (False, False, ["git", "version"]),
-        (False, True, "git version"),
-        (True, False, ["git", "version"]),
-        (True, True, "git version"),
+        # chdir_to_repo, shell, command, use_shell_impostor
+        (False, False, ["git", "version"], False),
+        (False, True, "git version", False),
+        (False, True, "git version", True),
+        (True, False, ["git", "version"], False),
+        (True, True, "git version", False),
+        (True, True, "git version", True),
     )
     @with_rw_directory
     def test_it_executes_git_not_from_cwd(self, rw_dir, case):
-        chdir_to_repo, shell, command = case
+        chdir_to_repo, shell, command, use_shell_impostor = case
 
         repo = Repo.init(rw_dir)
 
@@ -160,7 +178,16 @@ def test_it_executes_git_not_from_cwd(self, rw_dir, case):
             impostor_path.write_text("#!/bin/sh\n", encoding="utf-8")
             os.chmod(impostor_path, 0o755)
 
-        with cwd(rw_dir) if chdir_to_repo else contextlib.nullcontext():
+        if use_shell_impostor:
+            shell_name = "cmd.exe" if os.name == "nt" else "sh"
+            shutil.copy(impostor_path, Path(rw_dir, shell_name))
+
+        with contextlib.ExitStack() as stack:
+            if chdir_to_repo:
+                stack.enter_context(cwd(rw_dir))
+            if use_shell_impostor:
+                stack.enter_context(_patch_out_env("ComSpec"))
+
             # Run the command without raising an exception on failure, as the exception
             # message is currently misleading when the command is a string rather than a
             # sequence of strings (it really runs "git", but then wrongly reports "g").