Merge branch 'issue_27168_2' into 'master'

app/controllers/concerns/issuable_collections.rb

app/models/ci/pipeline.rb

app/models/merge_request.rb

app/services/ci/create_pipeline_service.rb

changelogs/unreleased/issue_27168_2.yml

.../20170507205316_add_head_pipeline_id_to_merge_requests.rb

...0170508170547_add_head_pipeline_for_each_merge_request.rb

db/schema.rb

features/steps/project/merge_requests.rb

spec/controllers/projects/merge_requests_controller_spec.rb

spec/features/cycle_analytics_spec.rb

spec/features/issuables/issuable_list_spec.rb

...es/merge_requests/merge_immediately_with_pipeline_spec.rb

...tures/merge_requests/merge_when_pipeline_succeeds_spec.rb

spec/features/merge_requests/mini_pipeline_graph_spec.rb

...merge_requests/only_allow_merge_if_build_succeeds_spec.rb

spec/features/merge_requests/widget_spec.rb

spec/lib/gitlab/cycle_analytics/events_spec.rb

spec/lib/gitlab/import_export/all_models.yml

spec/lib/gitlab/import_export/safe_model_attributes.yml