Clear cached merge_ref_sha on reopen

In https://gitlab.com/gitlab-org/gitlab/-/merge_requests/41572, we
started caching `merge_ref_sha` when we clean up the merge request
refs.

It is possible to re-open a closed MR though. If we don't clear
the cached `merge_ref_sha`, `MergeRequest#merge_ref_head` will
always point to the cached one which can be stale.

This fixes that by clearing the cached value on reopen.

app/services/merge_requests/reopen_service.rb

@@ -16,6 +16,7 @@ module MergeRequests
         merge_request.update_project_counter_caches
         merge_request.cache_merge_request_closes_issues!(current_user)
         merge_request.cleanup_schedule&.destroy
+        merge_request.update_column(:merge_ref_sha, nil)
       end
 
       merge_request

changelogs/unreleased/pb-clear-merge-ref-sha-reopen.yml

+---
+title: Clear cached merge_ref_sha on reopen
+merge_request: 47747
+author:
+type: fixed

spec/services/merge_requests/reopen_service_spec.rb

@@ -24,6 +24,7 @@ RSpec.describe MergeRequests::ReopenService do
       before do
         allow(service).to receive(:execute_hooks)
         merge_request.create_cleanup_schedule(scheduled_at: Time.current)
+        merge_request.update_column(:merge_ref_sha, 'abc123')
 
         perform_enqueued_jobs do
           service.execute(merge_request)
@@ -48,6 +49,10 @@ RSpec.describe MergeRequests::ReopenService do
         expect(merge_request.reload.cleanup_schedule).to be_nil
       end
 
+      it 'clears the cached merge_ref_sha' do
+        expect(merge_request.reload.merge_ref_sha).to be_nil
+      end
+
       context 'note creation' do
         it 'creates resource state event about merge_request reopen' do
           event = merge_request.resource_state_events.last