OperationRequestCompleter ignores CommandArgument#canAppearNext in choosing ValueCompleter when there's more then one indexed arguments

If there are two or more indexed CommandArguments that can be used to complete user entry at the same index, the first one will always be chosen - even if it clashes with previous arguments.

For example:
patch rollback ..<TAB>
produces a list of directories - this is because PatchHandler registers multiple CommandArguments with index 1 and in this case a completer for patch apply is the first one matched.