Prevent that an instance of PushCommand is reused

We checked if the command instance is called but missed to set the flag
tracking if it actually was used.

Change-Id: I0f9fb85c47945a4d91eab01ec5e1abc1ab83332a

org.eclipse.jgit/src/org/eclipse/jgit/api/PushCommand.java

@@ -98,6 +98,7 @@ public Iterable<PushResult> call() throws GitAPIException,
 			InvalidRemoteException,
 			org.eclipse.jgit.api.errors.TransportException {
 		checkCallable();
+		setCallable(false);
 
 		ArrayList<PushResult> pushResults = new ArrayList<>(3);