Implement instance USE_SHELL lookup in __getattr__

This is with the intention of making it so that Git.USE_SHELL is
unittest.mock.patch patchable. However, while this moves in the
right direction, it can't be done this way, as the attribute is
found to be absent on the class, so when unittest.mock.patch
unpatches, it tries to delete the attribute it has set, which
fails due to the metaclass's USE_SHELL instance property having no
deleter.

Author: EliahKagan

Diff for: git/cmd.py

@@ -399,28 +399,25 @@ def __setstate__(self, d: Dict[str, Any]) -> None:
 
     _USE_SHELL: bool = False
 
-    @property
-    def USE_SHELL(self) -> bool:
-        """Deprecated. If set to ``True``, a shell will be used to execute git commands.
+    USE_SHELL: bool
+    """Deprecated. If set to ``True``, a shell will be used to execute git commands.
 
-        Prior to GitPython 2.0.8, this had a narrow purpose in suppressing console
-        windows in graphical Windows applications. In 2.0.8 and higher, it provides no
-        benefit, as GitPython solves that problem more robustly and safely by using the
-        ``CREATE_NO_WINDOW`` process creation flag on Windows.
+    Prior to GitPython 2.0.8, this had a narrow purpose in suppressing console windows
+    in graphical Windows applications. In 2.0.8 and higher, it provides no benefit, as
+    GitPython solves that problem more robustly and safely by using the
+    ``CREATE_NO_WINDOW`` process creation flag on Windows.
 
-        Code that uses ``USE_SHELL = True`` or that passes ``shell=True`` to any
-        GitPython functions should be updated to use the default value of ``False``
-        instead. ``True`` is unsafe unless the effect of shell expansions is fully
-        considered and accounted for, which is not possible under most circumstances.
+    Code that uses ``USE_SHELL = True`` or that passes ``shell=True`` to any GitPython
+    functions should be updated to use the default value of ``False`` instead. ``True``
+    is unsafe unless the effect of shell expansions is fully considered and accounted
+    for, which is not possible under most circumstances.
 
-        See:
+    See:
 
-        - :meth:`Git.execute` (on the ``shell`` parameter).
-        - https://github.com/gitpython-developers/GitPython/commit/0d9390866f9ce42870d3116094cd49e0019a970a
-        - https://learn.microsoft.com/en-us/windows/win32/procthread/process-creation-flags
-        """
-        _warn_use_shell(False)
-        return self._USE_SHELL
+    - :meth:`Git.execute` (on the ``shell`` parameter).
+    - https://github.com/gitpython-developers/GitPython/commit/0d9390866f9ce42870d3116094cd49e0019a970a
+    - https://learn.microsoft.com/en-us/windows/win32/procthread/process-creation-flags
+    """
 
     _git_exec_env_var = "GIT_PYTHON_GIT_EXECUTABLE"
     _refresh_env_var = "GIT_PYTHON_REFRESH"
@@ -921,6 +918,11 @@ def __getattr__(self, name: str) -> Any:
         """
         if name.startswith("_"):
             return super().__getattribute__(name)
+
+        if name == "USE_SHELL":
+            _warn_use_shell(False)
+            return self._USE_SHELL
+
         return lambda *args, **kwargs: self._call_process(name, *args, **kwargs)
 
     def set_persistent_git_options(self, **kwargs: Any) -> None: