Don't call `#uniq` on a relation
When there was no project, no search, and no current user or author
param, the AutocompleteController would call
#uniq! on a relation
instead of an array. This performed the less-efficient
when it wasn't even needed (because the query wouldn't return duplicates
anyway - duplicates were only added by putting a user on top of the
Makes the worst case for https://gitlab.com/gitlab-org/gitlab-ce/issues/27085 better.