Add toString for TrackingRefUpdate

Makes it much easier to debug the results of
OperationResult#getTrackingRefUpdates (which otherwise requires digging
into a TreeMap structure).

Change-Id: I90da5385ee47c441404728f252eb3a100c48ee1c

org.eclipse.jgit/src/org/eclipse/jgit/transport/TrackingRefUpdate.java

@@ -193,4 +193,21 @@ public class TrackingRefUpdate {
 			}
 		}
 	}
+
+	@Override
+	public String toString() {
+		StringBuilder sb = new StringBuilder();
+		sb.append("TrackingRefUpdate[");
+		sb.append(remoteName);
+		sb.append(" -> ");
+		sb.append(localName);
+		if (forceUpdate)
+			sb.append(" (forced)");
+		sb.append(" ");
+		sb.append(oldObjectId == null ? "" : oldObjectId.abbreviate(7).name());
+		sb.append("..");
+		sb.append(newObjectId == null ? "" : newObjectId.abbreviate(7).name());
+		sb.append("]");
+		return sb.toString();
+	}
 }