sequencer: use return value of oidset_insert()

oidset_insert() returns 1 if the object ID is already in the set and
doesn't add it again, or 0 if it hadn't been present.  Make use of that
fact instead of checking with an extra oidset_contains() call.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Acked-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.c

@@ -4131,9 +4131,7 @@ static int make_script_with_merges(struct pretty_print_context *pp,
 			struct object_id *oid = &parent->item->object.oid;
 			if (!oidset_contains(&interesting, oid))
 				continue;
-			if (!oidset_contains(&child_seen, oid))
-				oidset_insert(&child_seen, oid);
-			else
+			if (oidset_insert(&child_seen, oid))
 				label_oid(oid, "branch-point", &state);
 		}