Only show diff between submissions in the same contest

We get this case for jury imports where the same team DOMjudge
uploads in multiple contests (dress + finals) and the first "finals" submission
would show a diff with the last jury submission of the dress contest.

We could also have this if a team is in 2 contests at the same time but
although in theory possible this shouldn't happen in practice.

Author: vmcj

webapp/src/Controller/Jury/SubmissionController.php

@@ -889,10 +889,12 @@ public function sourceAction(
                 ->andWhere('s.problem = :probid')
                 ->andWhere('s.language = :langid')
                 ->andWhere('s.submittime < :submittime')
+                ->andWhere('s.contest = :contest')
                 ->setParameter('teamid', $submission->getTeam())
                 ->setParameter('probid', $submission->getProblem())
                 ->setParameter('langid', $submission->getLanguage())
                 ->setParameter('submittime', $submission->getSubmittime())
+                ->setParameter('contest', $submission->getContest())
                 ->orderBy('s.submittime', 'DESC')
                 ->setMaxResults(1)
                 ->getQuery()