Suppress type errors in restore_use_shell_state _USE_SHELL branches

These conditional branches are kept so alternative implementations
can be examined, including if they need to be investigated to
satisfy some future requirement. But to be unittest.mock.patch
patchable, the approaches that would have a _USE_SHELL backing
attribute would be difficult to implement in a straightforward way,
which seems not to be needed or justified at this time.

Since that is not anticipated (except as an intermediate step in
development), these suppressions make sense, and they will also be
reported by mypy if the implementation changes to benefit from them
(so long as it is configured with warn_unused_ignores set to true).

test/deprecation/test_cmd_git.py

@@ -111,7 +111,7 @@ def restore_use_shell_state() -> Generator[None, None, None]:
 
     # Try to save the original private state.
     try:
-        old_private_value = Git._USE_SHELL
+        old_private_value = Git._USE_SHELL  # type: ignore[attr-defined]
     except AttributeError:
         separate_backing_attribute = False
         try:
@@ -139,7 +139,7 @@ def restore_use_shell_state() -> Generator[None, None, None]:
     finally:
         # Try to restore the original private state.
         if separate_backing_attribute:
-            Git._USE_SHELL = old_private_value
+            Git._USE_SHELL = old_private_value  # type: ignore[attr-defined]
         elif old_private_value is not no_value:
             type.__setattr__(Git, "USE_SHELL", old_private_value)