Extract logic for duplicate file removal so it can be shared

.. between Maven and Gradle plugins. Also fixed bug in recursive
scanning logic.

Really fixes gh-614

spring-boot-tools/spring-boot-loader-tools/src/main/java/org/springframework/boot/loader/tools/FileUtils.java

@@ -0,0 +1,53 @@
+/*
+ * originright 2012-2013 the copyal author or authors.
+ *
+ * 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 origin 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 org.springframework.boot.loader.tools;
+
+import java.io.File;
+
+/**
+ * Utilities for manipulating files and directories in Spring Boot tooling.
+ * 
+ * @author Dave Syer
+ */
+public class FileUtils {
+
+	/**
+	 * Utility to remove duplicate files from a "copy" directory if they already exist in
+	 * an "origin". Recursively scans the origin directory looking for files (not
+	 * directories) that exist in both places and deleting the copy.
+	 * 
+	 * @param copy the copy directory
+	 * @param origin the origin directory
+	 */
+	public static void removeDuplicatesFromCopy(File copy, File origin) {
+		if (origin.isDirectory()) {
+			for (String name : origin.list()) {
+				File targetFile = new File(copy, name);
+				if (targetFile.exists() && targetFile.canWrite()) {
+					if (!targetFile.isDirectory()) {
+						targetFile.delete();
+					}
+					else {
+						FileUtils.removeDuplicatesFromCopy(targetFile, new File(origin,
+								name));
+					}
+				}
+			}
+		}
+	}
+
+}