compact: derive freed space from quota use before/after, fixes #5679

due to the way quota accounting is done, this is likely not
100% precise, but much better than selling the hints as the truth.

src/borg/repository.py

@@ -738,7 +738,7 @@ class Repository:
         if not self.compact:
             logger.debug('nothing to do: compact empty')
             return
-        freed_space = 0
+        quota_use_before = self.storage_quota_use
         index_transaction_id = self.get_index_transaction_id()
         segments = self.segments
         unused = []  # list of segments, that are not used anymore
@@ -778,7 +778,6 @@ class Repository:
                              segment, freeable_ratio * 100.0, freeable_space)
                 pi.show()
                 continue
-            freed_space += freeable_space  # this is what we THINK we can free
             segments.setdefault(segment, 0)
             logger.debug('compacting segment %d with usage count %d (freeable: %2.2f%% [%d bytes])',
                          segment, segments[segment], freeable_ratio * 100.0, freeable_space)
@@ -867,7 +866,8 @@ class Repository:
             pi.show()
         pi.finish()
         complete_xfer(intermediate=False)
-        logger.info('compaction freed about %s repository space.', format_file_size(freed_space))
+        quota_use_after = self.storage_quota_use
+        logger.info('compaction freed about %s repository space.', format_file_size(quota_use_before - quota_use_after))
         logger.debug('compaction completed.')
 
     def replay_segments(self, index_transaction_id, segments_transaction_id):