Fix Snippet#participants.

app/models/commit.rb

@@ -138,6 +138,7 @@ class Commit
     users = []
     users << author
     users << committer
+    
     mentions = []
     mentions << self.mentioned_users(current_user, project)
 

app/models/concerns/issuable.rb

@@ -122,6 +122,7 @@ module Issuable
     users = []
     users << author
     users << assignee if is_assigned?
+    
     mentions = []
     mentions << self.mentioned_users(current_user)
 

app/models/snippet.rb

@@ -91,6 +91,8 @@ class Snippet < ActiveRecord::Base
     users = []
     users << author
 
+    mentions = []
+    
     notes.each do |note|
       users << note.author
       mentions << note.mentioned_users(current_user)