Autocomplete suggestions: no filter suggested when typing more than one letter of the filter name

Created by: lguychard

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

Steps to reproduce:

1. Type in "r" in the search query field
2. Witness several filters are suggested (repo, repogroup, repohasfile, repohascommitafter):
3. Now type "re"

Expected behavior:

All four filters beginning with "re" are still suggested.

Actual behavior:

No filters are suggested:

However, if I type "repo", "regex" is no longer suggested, but all filters beginning with "repo" are still suggested.

Same when typing "fi" (expecting the file: filter to be suggested):