Reject keep values below -1

A keep below -1 made remove_old_backups() delete every backup, including the new one, and then crash.

tests/test_cli.py

@@ -191,6 +191,18 @@ ssh_keyfile=/keys/id
         assert FakeBackup.instances == []
         assert 'Both SSH host and SSH user are needed' in caplog.text
 
+    @pytest.mark.parametrize('config_keep, argv', [
+        ('keep=-2', []),
+        ('', ['-k', '-2']),
+    ])
+    def test_keep_below_minus_one(self, run_cli: RunCli, config: Callable[[str], str],
+                                  caplog: pytest.LogCaptureFixture, config_keep: str, argv: list[str]) -> None:
+        config_file = config(f'backup_dir=/backups\n{config_keep}')
+
+        assert run_cli('-c', config_file, *argv) == 6
+        assert FakeBackup.instances == []
+        assert 'Invalid number of backups to keep: -2' in caplog.text
+
     def test_bad_config(self, run_cli: RunCli, config: Callable[[str], str],
                         caplog: pytest.LogCaptureFixture) -> None:
         config_file = config('backup_dir=/backups\nkeep=abc')