Don't print "into HEAD" when merging refs/heads/master

When MergeMessageFormatter was given a symbolic ref HEAD which points to
refs/heads/master (which is the case when merging a branch in EGit), it
would result in a merge message like the following:

  Merge branch 'a' into HEAD

But it should print the following (as C Git does):

  Merge branch 'a'

The solution is to use the leaf ref when checking for refs/heads/master.

Change-Id: I28ae5713b7e8123a0176fc6d7356e469900e7e97

org.eclipse.jgit.test/tst/org/eclipse/jgit/merge/MergeMessageFormatterTest.java

 import org.eclipse.jgit.lib.ObjectId;
 import org.eclipse.jgit.lib.ObjectIdRef;
 import org.eclipse.jgit.lib.Ref;
+import org.eclipse.jgit.lib.SymbolicRef;
 import org.eclipse.jgit.lib.Ref.Storage;
 import org.eclipse.jgit.lib.RefUpdate;
 import org.eclipse.jgit.lib.SampleDataRepositoryTestCase;
 		String message = formatter.format(Arrays.asList(a), b);
 		assertEquals("Merge branch 'a' into b", message);
 	}
+
+	@Test
+	public void testIntoSymbolicRefHeadPointingToMaster() throws IOException {
+		Ref a = db.getRef("refs/heads/a");
+		Ref master = db.getRef("refs/heads/master");
+		SymbolicRef head = new SymbolicRef("HEAD", master);
+		String message = formatter.format(Arrays.asList(a), head);
+		assertEquals("Merge branch 'a'", message);
+	}
 }

org.eclipse.jgit/src/org/eclipse/jgit/merge/MergeMessageFormatter.java

 
 		sb.append(StringUtils.join(listings, ", "));
 
-		if (!target.getName().equals(Constants.R_HEADS + Constants.MASTER)) {
+		String targetName = target.getLeaf().getName();
+		if (!targetName.equals(Constants.R_HEADS + Constants.MASTER)) {
 			String targetShortName = Repository
 					.shortenRefName(target.getName());
 			sb.append(" into " + targetShortName);