Don't access logger in tests by nonpublic attribute

This changes a couple tests that access specific loggers to use the
logger name instead, as code outside GitPython should do.

test/test_git.py

@@ -133,7 +133,7 @@ def test_it_uses_shell_or_not_as_specified(self, case):
     def test_it_logs_if_it_uses_a_shell(self, case):
         """``shell=`` in the log message agrees with what is passed to `Popen`."""
         value_in_call, value_from_class = case
-        with self.assertLogs(cmd._logger, level=logging.DEBUG) as log_watcher:
+        with self.assertLogs(cmd.__name__, level=logging.DEBUG) as log_watcher:
             mock_safer_popen = self._do_shell_combo(value_in_call, value_from_class)
         self._assert_logged_for_popen(log_watcher, "shell", mock_safer_popen.call_args.kwargs["shell"])
 
@@ -143,7 +143,7 @@ def test_it_logs_if_it_uses_a_shell(self, case):
     )
     def test_it_logs_istream_summary_for_stdin(self, case):
         expected_summary, istream_argument = case
-        with self.assertLogs(cmd._logger, level=logging.DEBUG) as log_watcher:
+        with self.assertLogs(cmd.__name__, level=logging.DEBUG) as log_watcher:
             self.git.execute(["git", "version"], istream=istream_argument)
         self._assert_logged_for_popen(log_watcher, "stdin", expected_summary)