Add Object type to Comparable implements declaration

This removes unchecked warnings when a List of
AnyObjectId objects or any of its subclasses are
passed to Collections.sort such as in PackWriter

Change-Id: I806732cee24349c75c0357a655df55b070f2f213
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

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

@@ -57,7 +57,7 @@ import org.eclipse.jgit.util.NB;
  * with this instance can alter at any time, if this instance is modified to
  * represent a different object name.
  */
-public abstract class AnyObjectId implements Comparable {
+public abstract class AnyObjectId implements Comparable<Object> {
 
 	/**
 	 * Compare to object identifier byte sequences for equality.