[findBugs] Don't call toString() on a string

Change-Id: I0b91ffd473e2f14623ddf409e4f0f342c8941e42
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit.test/tst/org/eclipse/jgit/diff/DiffFormatterTest.java

 				+ "\\ No newline at end of file\n" + "+folder change\n"
 				+ "\\ No newline at end of file\n";
 
-		assertEquals(expected.toString(), actual);
+		assertEquals(expected, actual);
 	}
 
 	private static String makeDiffHeader(String pathA, String pathB,