MergeCommandTest: Use JUnit's assume to check preconditions

Using the assume method, instead of just returning, will cause the
test to be marked as skipped rather than passed on systems where
the precondition is not satisfied.

Change-Id: I13672371f6cd3c481a0a6247e0eaed3aac6d766e
Signed-off-by: David Pursehouse <david.pursehouse@sonymobile.com>

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

 import static org.junit.Assert.assertNull;
 import static org.junit.Assert.assertTrue;
 import static org.junit.Assert.fail;
+import static org.junit.Assume.assumeTrue;
 
 import java.io.File;
 import java.util.Iterator;
 
 	@Test
 	public void testFileModeMerge() throws Exception {
-		if (!FS.DETECTED.supportsExecute())
-			return;
 		// Only Java6
+		assumeTrue(FS.DETECTED.supportsExecute());
 		try (Git git = new Git(db)) {
 			writeTrashFile("mergeableMode", "a");
 			setExecutable(git, "mergeableMode", false);
 
 	@Test
 	public void testFileModeMergeWithDirtyWorkTree() throws Exception {
-		if (!FS.DETECTED.supportsExecute())
-			return;
 		// Only Java6 (or set x bit in index)
+		assumeTrue(FS.DETECTED.supportsExecute());
 
 		try (Git git = new Git(db)) {
 			writeTrashFile("mergeableButDirty", "a");