ObjectIdSubclassMap: Add isEmpty() method

This class behaves like a cross between a Set and a Map, sometimes
we might expect to use the method isEmpty() to test for size() == 0.
So implement it, reducing the surprise folks get when they are given
one of these objects.

Change-Id: I0d68e1243da8e62edf79c6ba4fd925f643e80a88
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

org.eclipse.jgit/src/org/eclipse/jgit/lib/ObjectIdSubclassMap.java

@@ -135,6 +135,11 @@ public int size() {
 		return size;
 	}
 
+	/** @return true if {@link #size()} is 0. */
+	public boolean isEmpty() {
+		return size == 0;
+	}
+
 	public Iterator<V> iterator() {
 		return new Iterator<V>() {
 			private int found;