added diff url for pull requests

Author: mabako

Git.hub/PullRequest.cs

@@ -40,6 +40,11 @@ public class PullRequest
         /// </summary>
         public DateTime UpdatedAt { get; private set; }
 
+        /// <summary>
+        /// URL for the unified diff file
+        /// </summary>
+        public string DiffUrl { get; private set; }
+
         /// <summary>
         /// Internally used URL for this pull request.
         ///