Don't write to the filesystem when trimming.

ChangeSet._trim may call ChangeSet.delete() several times, which incurs
a disk write every time. This creates a lot of overhead, especially when
lots of revisions are removed.

Instead, leave the responsibility to the caller of _trim to make sure
that the new content is written to disk. In this case, ChangeSet.save()
the only caller, already writes to disk.

topydo/lib/ChangeSet.py

@@ -110,7 +110,7 @@ class ChangeSet(object):
         self._write()
         self.close()
 
-    def delete(self, p_timestamp=None):
+    def delete(self, p_timestamp=None, p_write=True):
         """ Removes backup from the backup file. """
         timestamp = p_timestamp or self.timestamp
         index = self._get_index()
@@ -119,6 +119,8 @@ class ChangeSet(object):
             del self.backup_dict[timestamp]
             index.remove(index[[change[0] for change in index].index(timestamp)])
             self._save_index(index)
+
+            if p_write:
                 self._write()
         except KeyError:
             pass
@@ -143,12 +145,15 @@ class ChangeSet(object):
         """
         Removes oldest backups that exceed the limit configured in backup_count
         option.
+
+        Does not write back to file system, make sure to call self._write()
+        afterwards.
         """
         index = self._get_index()
         backup_limit = config().backup_count() - 1
 
         for changeset in index[backup_limit:]:
-            self.delete(changeset[0])
+            self.delete(changeset[0], p_write=False)
 
     def get_backup(self, p_todolist):
         """