Cancel autocomplete on "{" and "}"

Summary: Ref T10163. These are almost certainly not username/project characters, and are fairly likely to be @{...} Diviner references.

Test Plan: Typed @{..., no more autocomplete.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10163

Differential Revision: https://secure.phabricator.com/D15110