scorch zap panic if mergeFields() sees unsorted fields
scorch zap panic if mergeFields() sees unsorted fields

mergeFields depends on the fields from the various segments being
sorted for the fieldsSame comparison to work.

Of note, the 'fieldi > 1' guard skips the 0th field, which should
always be the '_id' field.

blevesearch/bleve,blevesearch/bleve,steveyen/bleve,steveyen/bleve