Mark this diff (or volume) to be kept in path.

def keep(self, diff):
        """ Mark this diff (or volume) to be kept in path. """
        path = self.extraKeys[diff]

        if not path.startswith("/"):
            logger.debug("Keeping %s", path)
            del self.extraKeys[diff]
            return

        # Copy into self.userPath, if not there already

        keyName = self._keyName(diff.toUUID, diff.fromUUID, path)
        newPath = os.path.join(self.userPath, os.path.basename(path))
        newName = self._keyName(diff.toUUID, diff.fromUUID, newPath)

        if not self._skipDryRun(logger)("Copy %s to %s", keyName, newName):
            self.bucket.copy_key(newName, self.bucket.name, keyName)