R1:14ebf662f30a

Don't show the autocompleter until the user types at least one character

Summary: Ref T10163. Activate on `@d`, not just `@`. Note that if you type `@d` and then press delete once so you're left with `@`, we stay active (and show the "type a username" hint).

Test Plan:
- Typed `@`, no completer.
- Typed `d`, got completer.
- Typed delete, still had completer, now showing hint prompt.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10163

Differential Revision: https://secure.phabricator.