Do not use MergeRequest#commits in define_pipelines_vars

MergeRequest#commits requires merge_request_diff to be present.
This is not the case when creating a new merge request.

app/controllers/projects/merge_requests_controller.rb

@@ -563,11 +563,8 @@ class Projects::MergeRequestsController < Projects::ApplicationController
 
   def define_pipelines_vars
     @pipelines = @merge_request.all_pipelines
-
-    if @pipelines.present? && @merge_request.commits.present?
-      @pipeline = @pipelines.first
-      @statuses = @pipeline.statuses.relevant
-    end
+    @pipeline = @merge_request.pipeline
+    @statuses = @pipeline.statuses.relevant if @pipeline.present?
   end
 
   def define_new_vars