Add a subpath routine to FileSignature.

Author: nedtwigg

lib/src/main/java/com/diffplug/spotless/FileSignature.java

@@ -119,4 +119,13 @@ public static String pathNativeToUnix(String pathNative) {
 	public static String pathUnixToNative(String pathUnix) {
 		return LineEnding.nativeIsWin() ? pathUnix.replace('/', '\\') : pathUnix;
 	}
+
+	/** Asserts that child is a subpath of root. and returns the subpath. */
+	public static String subpath(String root, String child) {
+		if (child.startsWith(root)) {
+			return child.substring(root.length());
+		} else {
+			throw new IllegalArgumentException("Expected '" + child + "' to start with '" + root + "'");
+		}
+	}
 }

testlib/src/test/java/com/diffplug/spotless/FileSignatureTest.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2016 DiffPlug
+ * Copyright 2016-2020 DiffPlug
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -56,4 +56,8 @@ private List<File> getTestFiles(final String[] paths) throws IOException {
 		return result;
 	}
 
+	@Test
+	public void testSubpath() {
+		assertThat(FileSignature.subpath("root/", "root/child")).isEqualTo("child");
+	}
 }