[FIX] runbot_merge: when looking for stageable PRs group by target

This is more of a sanity check as it normally should not be a factor:
labels generally contain the target name, and staging checks are
performed per-target so we're not mixing multiple targets anyway.

But let's say a third-party creates a fix-foo branch for A and a
fix-foo branch for B, we want to ensure they're not considered batched
together.

runbot_merge/models/pull_requests.py

@@ -337,6 +337,7 @@ class Branch(models.Model):
           AND pr.state != 'merged'
           AND pr.state != 'closed'
         GROUP BY
+            pr.target,
             CASE
                 WHEN pr.label SIMILAR TO '%%:patch-[[:digit:]]+'
                     THEN pr.id::text