database: Store directory path in 'data' of directory documents.

database: Store directory path in 'data' of directory documents.

We're planning to have mail documents refer to directory documents for
the path of the containing directory. To support this, we need the
path in the data, (since the path in the 'directory' term can be
irretrievable as it will be the SHA1 sum of the path for a very long
path).