Fix a bug in completion merging causing unwanted closing quotes.

When one completer doesn't have any candidates except a sole closing
quote, and the other one has something else with more context (i.e.
completed relatively to lower cursor position),
CommandArguments.mergeCompletions() should discard the sole closing
quote candidate.

Otherwise, if we for example try complete `"Wolf4` and one completer
knows for sure the completion is `"Wolf480pl"`, and the other completer
doesn't have idea, we'll end up with 2 propositions: `"Wolf480pl"` and
`"Wolf4"`.