Smarter revisions strategy, no revisions created unless already there or explicitly requested.

Save()
fontforge.activeFont().save()
  if there are revisions already, then new revisions are created.
  Otherwise no revisions.

Save("foo.sfd")
fontforge.activeFont().save("/tmp/norev.sfd")
fontforge.activeFont().save("/tmp/withrev.sfd")
  if there are revisions already, then new revisions are created.
  Otherwise no revisions.

Save("foo.sfd",32)
fontforge.activeFont().save("/tmp/norev.sfd",32)
fontforge.activeFont().save("/tmp/withrev.sfd",32)
  Revisions are created. if foo.sfd didn't exist then only foo.sfd
  exists after the call. If foo.sfd existed then it will be foo.sfd-01
  and a new foo.sfd written.