Merge "Change visibility of DirCacheTree#getObjectId to public"

org.eclipse.jgit/src/org/eclipse/jgit/dircache/DirCacheTree.java

@@ -249,7 +249,15 @@ public class DirCacheTree {
 		return children[i];
 	}
 
-	ObjectId getObjectId() {
+	/**
+	 * Get the tree's ObjectId.
+	 * <p>
+	 * If {@link #isValid()} returns false this method will return null.
+	 *
+	 * @return ObjectId of this tree or null.
+	 * @since 4.3
+	 */
+	public ObjectId getObjectId() {
 		return id;
 	}