Let users escape more easily from the autosuggester after typing "[" or "("

Summary:
Ref T13077. The autosuggester is a little too eager right now, and will eat carriage returns after typing [ if you never activate the tokenizer.

To fix this, try just canceling sooner. If that doesn't work, we might need to cancel more eagerly by testing to see if the tokenizer is actually open.

Test Plan: Typed [x]<return>, got my return instead of getting trapped by the autosuggester.

Maniphest Tasks: T13077

Differential Revision: https://secure.phabricator.com/D19110