Change visibility of DirCacheTree#getObjectId to public

Allow access to the ObjectId of a DirCacheTree if known for low level
integration code.

Change-Id: I6f05b10c9ac781f5e8b38af4a19e653313c91fa8
Signed-off-by: Philipp Marx <smigfu@googlemail.com>

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

 		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;
 	}