Unrecoverable after filecount assertion fail

[mfitz]

Near the end of a full backup, duplicity crashed running out of memory.

When running it again, this assertion was getting triggered in manifest.py.

assert filecount =len(self.files_changed))

The partially written list of files in the manifest was shorter than the filecount, so duplicity could not handle this. I had to remove the filecount and the file listing by hand so that it could resume the backup at the last volume.