Optimize Ci::Pipeline.latest query

Since we already know which ref we want, we could filter it out first.

Feedback:
https://gitlab.com/gitlab-org/gitlab-ce/issues/26570#note_23423376

Closes #26570

app/models/ci/pipeline.rb

@@ -91,6 +91,7 @@ module Ci
     scope :latest, ->(ref = nil) do
       max_id = unscope(:select)
         .select("max(#{quoted_table_name}.id)")
+        .where(ref: ref)
         .group(:ref, :sha)
       relation = ref ? where(ref: ref) : self