Fix slow completion on method chain of huge arrays and hashes (size >= 10000000)

[tompng]

These completions took about 0.5sec

irb(main):001> a = [1] * 10000000; a.size
=> 10000000
irb(main):002* a.first.

irb(main):001> h = 10000000.times.to_h { [it, it] }; h.size
=> 10000000
irb(main):002* h.keys.