Fix - in cross tab completition all choices from one file were joined in...

Error was introduced with use thread for completions.
Before:
```ruby
choices = []
['a','b'].each do |name|
  choices += [name, 'c'].flatten
end
choices.inspect # => ['a', 'c', 'b', 'c']
```
After threads:
```ruby
choices = ['a','b'].map do |name|\
  [name, 'c'].flatten
end
choices.inspect # => [["a", "c"], ["b", "c"]]