Merge branch 'fix-group-mention' into 'master'

Fix mentioning of private groups.

`issue.participants` would mistakenly not send the `current_user` to `mentioned_users`, which means it would search for referenced groups and users as if the user was signed out. Since signed out users don't have access to private groups (groups without public projects) and since you can't reference a group you don't have access to (since this would leak its existence), the group reference wasn't found and the group members weren't notified.

Addresses internal issue https://dev.gitlab.org/gitlab/gitlabhq/issues/2326.

See merge request !654

CHANGELOG

@@ -58,6 +58,7 @@ v 7.11.0 (unreleased)
   - Fix automatic blocking of auto-created users from Active Directory.
   - Call merge request web hook for each new commits (Arthur Gautier)
   - Use SIGKILL by default in Sidekiq::MemoryKiller
+  - Fix mentioning of private groups.
 
 v 7.10.2
   - Fix CI links on MR page

app/models/concerns/mentionable.rb

@@ -42,10 +42,10 @@ module Mentionable
     SystemNoteService.cross_reference_exists?(target, local_reference)
   end
 
-  def mentioned_users(current_user = nil, p = project)
+  def mentioned_users(current_user = nil)
     return [] if mentionable_text.blank?
 
-    ext = Gitlab::ReferenceExtractor.new(p, current_user)
+    ext = Gitlab::ReferenceExtractor.new(self.project, current_user)
     ext.analyze(mentionable_text)
     ext.users.uniq
   end

app/models/concerns/participable.rb

@@ -40,7 +40,7 @@ module Participable
       meth = method(attr)
 
       value = 
-        if meth.arity == 1
+        if meth.arity == 1 || meth.arity == -1
           meth.call(current_user)
         else
           meth.call