Memoize MergeRequest#rebase_in_progress? to prevent N+1 queries in Gitaly

MergeRequest#rebase_in_progress? and MergeRequest#rebase_path were called
twice each time per request. This memoization helps reduce the overall number
of queries.

See https://gitlab.com/gitlab-org/gitlab-ee/-/jobs/51616319 as a failing
job.

Closes gitlab-org/gitlab-ee#4857

app/models/merge_request.rb

@@ -158,11 +158,13 @@ class MergeRequest < ActiveRecord::Base
   end
 
   def rebase_in_progress?
+    strong_memoize(:rebase_in_progress) do
       # The source project can be deleted
-    return false unless source_project
+      next false unless source_project
 
       source_project.repository.rebase_in_progress?(id)
     end
+  end
 
   # Use this method whenever you need to make sure the head_pipeline is synced with the
   # branch head commit, for example checking if a merge request can be merged.