do not find fileid of a point if already known

main/OpenCover.Framework/Persistance/BasePersistance.cs

@@ -393,7 +393,7 @@ from method in @class.Methods.Where(x => !x.ShouldSerializeSkippedDueTo())
 
         private static void MapFileReferences(IEnumerable<IDocumentReference> points, IDictionary<string, uint> filesDictionary)
         {
-            foreach (var pt in points)
+            foreach (var pt in points.Where(p => p.FileId == 0))
             {
                 uint fileid;
                 filesDictionary.TryGetValue(pt.Document ?? "", out fileid);