Use porcelain commit lookup method on CI::CreatePipelineService

Before we were using a "plumbing" Gitlab::Git method that does not go
through Gitaly migration checking.

app/services/ci/create_pipeline_service.rb

@@ -65,7 +65,7 @@ module Ci
       project.pipelines
         .where(ref: pipeline.ref)
         .where.not(id: pipeline.id)
-        .where.not(sha: project.repository.sha_from_ref(pipeline.ref))
+        .where.not(sha: project.commit(pipeline.ref).try(:id))
         .created_or_pending
     end

changelogs/unreleased/ci-pipeline-commit-lookup.yml

+---
+title: Use porcelain commit lookup method on CI::CreatePipelineService
+merge_request: 17911
+author:
+type: fixed

lib/gitlab/git/repository.rb

@@ -516,10 +516,6 @@ module Gitlab
         end
       end
-      def sha_from_ref(ref)
-        rev_parse_target(ref).oid
-      end
-
       # Return the object that +revspec+ points to.  If +revspec+ is an
       # annotated tag, then return the tag's target instead.
       def rev_parse_target(revspec)
@@ -2409,6 +2405,10 @@ module Gitlab
       def rev_list_param(spec)
         spec == :all ? ['--all'] : spec
       end
+
+      def sha_from_ref(ref)
+        rev_parse_target(ref).oid
+      end
     end
   end
 end