Autocomplete suggestions: negated filters are not suggested

Created by: lguychard

• Sourcegraph version: sourcegraph/server:3.10.0-rc.2

Steps to reproduce:

1. Type a - in the search field

Expected behavior:

Negated filters (-repo:, -repohasfile:, -file:, -lang:) are shown. When one of these filters is selected, appropriate suggestions are shown (repos, files, languages...)

Actual behavior:

No suggestions are shown: