Keep only rsync's summary lines when not verbose

Every output line was kept in memory although only the last two were logged, and fewer than two lines (e.g. with --rsync-options q) raised an IndexError.

simple_backup/simple_backup.py

@@ -24,6 +24,7 @@ import re
 import shlex
 import sys
 import warnings
+from collections import deque
 from collections.abc import Callable, Iterator
 from datetime import datetime
 from functools import wraps
@@ -774,7 +775,8 @@ class Backup:
             assert p.stdin is not None and isinstance(p.stdout, io.BufferedIOBase)
             p.stdin.close()
 
-            output = []
+            # Without -v, only the summary (the last two lines) is logged
+            output: list[str] | deque[str] = [] if self._verbose else deque(maxlen=2)
 
             try:
                 for r in _read_records(p.stdout):
@@ -790,8 +792,9 @@ class Backup:
             if self._verbose:
                 logger.info('rsync: %s', output)
             else:
-                logger.info('rsync: %s', output[-2])
-                logger.info('rsync: %s', output[-1])
+                # May be fewer than two lines, e.g. with --rsync-options q
+                for r in output:
+                    logger.info('rsync: %s', r)
         else:
             self._returncode_log(returncode)