BibUpload: --append only new fields

* When bibupload --append mode is used never create
  duplicate fields. Two fields are considered identical
  if they have the same indicators and exactly the same
  subfields in the same order.
  (closes #1440)

Reported-by: Florian Schwennsen <schwenn@mail.desy.de>
Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>
Tested-by: Tibor Simko <tibor.simko@cern.ch>