Bug 89020 [Undo] Operation throws assertion failure if undo limit is set to 0
I20050324-0800 Unfortunately existing clients of text's undo manager were allowed to set any number without failing. This is no longer possible: an assertion failure occurrs if the limit is smaller than 1. I will add code on my side that will handle the negative numbers but the operation history must allow 0.
bundles/org.eclipse.core.commands/src/org/eclipse/core/commands/operations/DefaultOperationHistory.java
* 
	 * @see org.eclipse.core.commands.operations.IOperationHistory#openOperation(org.eclipse.core.commands.operations.ICompositeOperation)
	 */
	public void openOperation(ICompositeOperation operation, int mode) {
		if (openComposite != null) {
			/* unexpected nesting of operations.  The original composite operation
			   will be closed and will be considered unsuccessful.  The third parameter
			   does not matter in this case since the operation is a failure. */
			if (DEBUG_OPERATION_HISTORY_UNEXPECTED) {
				System.out.print("OPERATIONHISTORY >>> Open operation called while another operation is open.  old: ");  
				System.out.print(openComposite);
				System.out.print("new:  "); 
				System.out.print(operation);
				System.out.println();
			}
			closeOperation(false, false, EXECUTE);	
		} 
		openComposite = operation;
		if (DEBUG_OPERATION_HISTORY_OPENOPERATION) {
			System.out.print("OPERATIONHISTORY >>> Opening operation ");  
			System.out.print(openComposite);
			System.out.println();
		}
		if (mode == EXECUTE) {
			notifyAboutToExecute(openComposite);
		}
	}
	
	/*
	 * (non-Javadoc)