CommitAndLogCommandTest: Use assumeFalse to skip test on Windows

Use JUnit's assumeFalse method to cause the test to skip when
run on Windows.

Change-Id: I3f59440cfe62c37c127e381052b60471fbe8ec5e
Signed-off-by: David Pursehouse <david.pursehouse@sonymobile.com>

org.eclipse.jgit.test/tst/org/eclipse/jgit/api/CommitAndLogCommandTest.java

 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertTrue;
 import static org.junit.Assert.fail;
+import static org.junit.Assume.assumeFalse;
 
 import java.io.File;
 import java.io.IOException;
 
 	@Test
 	public void testModeChange() throws IOException, GitAPIException {
-		if (System.getProperty("os.name").startsWith("Windows"))
-			return; // SKIP
+		assumeFalse(System.getProperty("os.name").startsWith("Windows"));// SKIP
 		try (Git git = new Git(db)) {
 			// create file
 			File file = new File(db.getWorkTree(), "a.txt");