Make Git.execute a bit simpler and very slightly more robust

This covers the rare unpatched python/cpython#101283 case the
previous commit added tests for, that only applies in the unusual
situation that the ComSpec environment variable is unset and an old
build (but this includes downloadable builds and current
actions/setup-python builds) of Python <=3.9 for Windows is in use.

The main benefit of this change is really to slightly simplify the
code under test. (It might even be justified to remove the
use_shell_impostor=True test cases at some point.)

git/cmd.py

@@ -988,25 +988,22 @@ def execute(
         if shell is None:
             shell = self.USE_SHELL
 
-        cmd_not_found_exception = FileNotFoundError
-        maybe_patch_caller_env = contextlib.nullcontext()
-
         if os.name == "nt":
+            cmd_not_found_exception = OSError
             if kill_after_timeout is not None:
                 raise GitCommandError(
                     redacted_command,
                     '"kill_after_timeout" feature is not supported on Windows.',
                 )
-
-            cmd_not_found_exception = OSError
-
-            # Search PATH, but do not search CWD. The "1" can be any value.
+            # Search PATH but not CWD. The "1" can be any value. We'll patch just before
+            # the Popen call and unpatch just after, or we get a worse race condition.
+            maybe_patch_caller_env = patch_env("NoDefaultCurrentDirectoryInExePath", "1")
             if shell:
-                # If the direct subprocess is a shell, this must go in its environment.
+                # Modify the direct shell subprocess's own search behavior accordingly.
                 env["NoDefaultCurrentDirectoryInExePath"] = "1"
-            else:
-                # If we're not using a shell, the variable goes in our own environment.
-                maybe_patch_caller_env = patch_env("NoDefaultCurrentDirectoryInExePath", "1")
+        else:
+            cmd_not_found_exception = FileNotFoundError
+            maybe_patch_caller_env = contextlib.nullcontext()
         # END handle
 
         stdout_sink = PIPE if with_stdout else getattr(subprocess, "DEVNULL", None) or open(os.devnull, "wb")