Fix bug in divergence timeline check (#2221)

Patroni was falsely assuming that timelines have diverged.
For pg_rewind it didn't create any problem, but if pg_rewind is not allowed and the `remove_data_directory_on_diverged_timelines` is set, it resulted in reinitializing the former leader.

Close https://github.com/zalando/patroni/issues/2220

patroni/postgresql/rewind.py

@@ -218,6 +218,7 @@ class Rewind(object):
                         need_rewind = True
                     else:
                         need_rewind = switchpoint != self._get_checkpoint_end(local_timeline, local_lsn)
+                    break
                 elif parent_timeline > local_timeline:
                     need_rewind = True
                     break

tests/test_rewind.py

@@ -141,6 +141,9 @@ class TestRewind(BaseTestPostgresql):
         self.leader = self.leader.member
         self.assertFalse(self.r.rewind_or_reinitialize_needed_and_possible(self.leader))
         mock_check_leader_is_not_in_recovery.return_value = True
+        self.assertFalse(self.r.rewind_or_reinitialize_needed_and_possible(self.leader))
+        self.r.trigger_check_diverged_lsn()
+        with patch.object(MockCursor, 'fetchone', Mock(side_effect=[('', 3, '0/0'), ('', b'4\t0/40159C0\tn\n')])):
             self.assertTrue(self.r.rewind_or_reinitialize_needed_and_possible(self.leader))
         self.r.reset_state()
         self.r.trigger_check_diverged_lsn()