Fix 500 when selecting a mirror user

When selecting a mirror user, we do a permissions check, which means
that `@users` is an array, not an ActiveRecord relation.

We also can't use `Array#delete`, because it uses `Object#equal?`, while
we need to be able to take advantage of ActiveRecord objects overriding
`#eql?` and `#hash?` (as `Array#uniq` uses a hash internally).

app/controllers/autocomplete_controller.rb

@@ -18,8 +18,7 @@ class AutocompleteController < ApplicationController
     if params[:search].blank?
       # Include current user if available to filter by "Me"
       if params[:current_user].present? && current_user
-        @users = @users.where.not(id: current_user.id)
-        @users = [current_user, *@users]
+        @users = [current_user, *@users].uniq
       end
 
       if params[:author_id].present?

changelogs/unreleased-ee/29316-can-t-select-a-mirror-user-ee.yml

+---
+title: Fix 500 error when selecting a mirror user
+merge_request:
+author:

spec/controllers/autocomplete_controller_spec.rb

@@ -61,6 +61,18 @@ describe AutocompleteController do
         it { expect(body.size).to eq 2 }
         it { expect(body.map { |user| user["username"] }).to match_array([user.username, user2.username]) }
       end
+
+      describe "GET #users that can push to protected branches, including the current user" do
+        before do
+          get(:users, project_id: project.id, push_code_to_protected_branches: true, current_user: true)
+        end
+
+        let(:body) { JSON.parse(response.body) }
+
+        it { expect(body).to be_kind_of(Array) }
+        it { expect(body.size).to eq 1 }
+        it { expect(body.first["username"]).to eq user.username }
+      end
     end
 
     context 'group members' do