diff: restore compatibility with upsource plugin

platform/diff-impl/src/com/intellij/diff/contents/DiffPsiFileSupport.java

@@ -0,0 +1,27 @@
+/*
+ * Copyright 2000-2017 JetBrains s.r.o.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.intellij.diff.contents;
+
+import com.intellij.codeInsight.daemon.OutsidersPsiFileSupport;
+import com.intellij.openapi.vfs.VirtualFile;
+import org.jetbrains.annotations.Nullable;
+
+@Deprecated
+public class DiffPsiFileSupport {
+  public static boolean isDiffFile(@Nullable VirtualFile file) {
+    return OutsidersPsiFileSupport.isDiffFile(file);
+  }
+}