ObjectIdSubclassMap: Correct Iterator to throw NoSuchElementException

The Iterator contract says next() shall throw NoSuchElementException
if there are no more items remaining in the iteration.  We got this
wrong when I originally wrote the implementation, so fix it.

Change-Id: Iea25e6569ead5c8b3128b8a368c5b2caebec7ecc
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

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

@@ -46,6 +46,7 @@
 package org.eclipse.jgit.lib;
 
 import java.util.Iterator;
+import java.util.NoSuchElementException;
 
 /**
  * Fast, efficient map specifically for {@link ObjectId} subclasses.
@@ -158,7 +159,7 @@ public V next() {
 						return v;
 					}
 				}
-				throw new IllegalStateException();
+				throw new NoSuchElementException();
 			}
 
 			public void remove() {