Rename `MergeRequest#pipeline` to `head_pipeline`

app/controllers/projects/merge_requests_controller.rb

app/models/ci/pipeline.rb

app/models/merge_request.rb

app/services/merge_requests/base_service.rb

app/views/projects/issues/_merge_requests.html.haml

app/views/projects/merge_requests/_merge_request.html.haml

app/views/projects/merge_requests/widget/_show.html.haml

lib/api/merge_requests.rb

spec/models/merge_request_spec.rb

spec/requests/api/merge_requests_spec.rb

.../merge_requests/add_todo_when_build_fails_service_spec.rb

.../merge_requests/merge_when_build_succeeds_service_spec.rb