transaction: apply checkambig=True only on limited files for similarity Now, transaction can determine whether avoidance of file stat ambiguity is needed for each files, by blacklist "checkambigfiles". For similarity to truncation in _playback(), this patch apply checkambig=True only on limited files in code paths below. - restoring files by util.copyfile(), in _playback() (checkambigfiles itself is examined at first, because it as a keyword argument might be None) - writing files at finalization of transaction, in _generatefiles() This patch reduces cost of checking stat at writing out and restoring files, which aren't filecache-ed.