Geo: Merge requests on Secondary should not check mergeable status

Merged Gabriel Mazetto requested to merge patch/geo-mergerequest-500 into master

In a secondary Geo (#76 (closed)) node, database is read-only.

If you update your merge request and don't visit the primary node, you would end up with an unchecked merge_status. We should not try to solve that in a secondary node, as this will lead to a 500 error because we can't update data on database.

This MR Fixes that:

Started GET "/root/test-project/merge_requests/1" for 189.101.232.69 at 2016-04-28 11:37:28 -0400
Processing by Projects::MergeRequestsController#show as HTML
  Parameters: {"namespace_id"=>"root", "project_id"=>"test-project", "id"=>"1"}
Completed 500 Internal Server Error in 372ms (ActiveRecord: 26.9ms)

ActionView::Template::Error (PG::ReadOnlySqlTransaction: ERROR:  cannot execute UPDATE in a read-only transaction
: UPDATE "merge_requests" SET "merge_status" = $1 WHERE "merge_requests"."id" = $2):
    39:     = render "projects/merge_requests/show/how_to_merge"
    40:     = render "projects/merge_requests/widget/show.html.haml"
    41:
    42:     - if @merge_request.source_branch_exists? && @merge_request.mergeable? && @merge_request.can_be_merged_by?(current_user)
    43:       .light.prepend-top-default
    44:         You can also accept this merge request manually using the
    45:         = succeed '.' do
  app/models/merge_request.rb:122:in `call'
  app/models/merge_request.rb:122:in `block (2 levels) in <class:MergeRequest>'
  app/models/merge_request.rb:272:in `check_if_can_be_merged'
  app/models/merge_request.rb:297:in `mergeable?'
  app/views/projects/merge_requests/_show.html.haml:42:in `_app_views_projects_merge_requests__show_html_haml__3632812159035673711_50958700'
  app/views/projects/merge_requests/show.html.haml:1:in `_app_views_projects_merge_requests_show_html_haml___4126596802437062695_51012840'