keyring: invert the lookup order in this query

Apparently, for some reason, Person.objects.get(fprs__fpr=self.fpr)
yields MultipleObjectsReturned in case of unknown fingerprint.

Signed-off-by: Mattia Rizzolo <mattia@debian.org>

keyring/git_ops.py

@@ -485,8 +485,8 @@ class RemoveDM(Remove):
                 pass
 
         try:
-            persons["fpr"] = bmodels.Person.objects.get(fprs__fpr=self.fpr)
-        except bmodels.Person.DoesNotExist:
+            persons["fpr"] = bmodels.Fingerprint.objects.get(fpr=self.fpr).person
+        except bmodels.Fingerprint.DoesNotExist:
             pass
 
         person = self._get_consistent_person(persons)