Merge branch 'access-check-when-signed-out' into 'master'

Fix push access check when not signed in.

Was causing 500 on MR, Blob, Tree pages.

Fixes https://github.com/gitlabhq/gitlabhq/issues/8637 and https://github.com/gitlabhq/gitlabhq/issues/8813.

See merge request !1542

lib/gitlab/git_access.rb

@@ -6,6 +6,8 @@ module Gitlab
     attr_reader :params, :project, :git_cmd, :user
 
     def self.can_push_to_branch?(user, project, ref)
+      return false unless user
+      
       if project.protected_branch?(ref)  &&
           !(project.developers_can_push_to_protected_branch?(ref) && project.team.developer?(user))
         user.can?(:push_code_to_protected_branches, project)