Fix incremental fetch problem for p2o

[theSalt]

For p2o.py, even if --no_incremental is not set, all items of raw index will still be fetched for enrich index. I found the problem when I use p2o.py to fetch git repo if from-date is not set, perceval only retrieve the incremental git log, but all index will be re-created for enrich git index.

Because no last_enrich_date is passed to enrich_backend() by p2o