Use a stable sort for hitobjects

osu.Game/Beatmaps/Formats/LegacyBeatmapDecoder.cs

@@ -56,7 +56,7 @@ protected override void ParseStreamInto(StreamReader stream, Beatmap beatmap)
 
             // objects may be out of order *only* if a user has manually edited an .osu file.
             // unfortunately there are ranked maps in this state (example: https://osu.ppy.sh/s/594828).
-            this.beatmap.HitObjects.Sort((x, y) => x.StartTime.CompareTo(y.StartTime));
+            this.beatmap.HitObjects = this.beatmap.HitObjects.OrderBy(h => h.StartTime).ToList();
 
             foreach (var hitObject in this.beatmap.HitObjects)
                 hitObject.ApplyDefaults(this.beatmap.ControlPointInfo, this.beatmap.BeatmapInfo.BaseDifficulty);