Fix harvests2json to skip generated documents.

Summary:
 - processing file <name>.harvest will be skipped if
 <name>.harvest.json is present.

Test Plan:
./bin/mws-index -I data/zbl/ -o data/zbl-index
./bin/harvests2json -H data/zbl -I data/zbl-index
./bin/harvests2json -H data/zbl -I data/zbl-index

Reviewers: radu.hambasan

Reviewed By: radu.hambasan

Differential Revision: http://phab.code4fun.de/D79