Backport EE changes to UserExtractor to CE

This backports the changes EE made to Gitlab::UserExtractor, removing
the need for an EE specific module.

lib/gitlab/user_extractor.rb

@@ -11,11 +11,14 @@ module Gitlab
     USERNAME_REGEXP = User.reference_pattern
     def initialize(text)
-      @text = text
+      # EE passes an Array to `text` in a few places, so we want to support both
+      # here.
+      @text = Array(text).join(' ')
     end
     def users
       return User.none unless @text.present?
+      return User.none if references.empty?
       @users ||= User.from_union(union_relations)
     end

spec/lib/gitlab/user_extractor_spec.rb

@@ -38,6 +38,18 @@ describe Gitlab::UserExtractor do
       expect(extractor.users).to include(user)
     end
+
+    context 'input as array of strings' do
+      it 'is treated as one string' do
+        extractor = described_class.new(text.lines)
+
+        user_1 = create(:user, username: "USER-1")
+        user_4 = create(:user, username: "USER-4")
+        user_email = create(:user, email: 'user@gitlab.org')
+
+        expect(extractor.users).to contain_exactly(user_1, user_4, user_email)
+      end
+    end
   end
   describe '#matches' do
@@ -48,6 +60,14 @@ describe Gitlab::UserExtractor do
     it 'includes all mentioned usernames' do
       expect(extractor.matches[:usernames]).to contain_exactly('user-1', 'user-2', 'user-4')
     end
+
+    context 'input has no matching e-mail or usernames' do
+      it 'returns an empty list of users' do
+        extractor = described_class.new('My test')
+
+        expect(extractor.users).to be_empty
+      end
+    end
   end
   describe '#references' do