repository: Increase index size for repo version 2

A compressed index is only about one third the size of an uncompressed
one. Thus increase the number of entries in an index to avoid cluttering
the repository with small indexes.

cmd/restic/integration_test.go

@@ -1470,7 +1470,7 @@ func TestRebuildIndexAlwaysFull(t *testing.T) {
 	defer func() {
 		repository.IndexFull = indexFull
 	}()
-	repository.IndexFull = func(*repository.Index) bool { return true }
+	repository.IndexFull = func(*repository.Index, bool) bool { return true }
 	testRebuildIndex(t, nil)
 }