Improve performance of the /projects/:id/repository/branches API endpoint

Mitigate a N+1 requests to Gitaly problem. Still one left.
Signed-off-by: Rémy Coutable <remy@rymai.me>

changelogs/unreleased/37442-api-branches-id-repository-branches-is-calling-gitaly-n-1-times-per-request.yml

+---
+title: Improve performance of the /projects/:id/repository/branches API endpoint
+merge_request: 15215
+author:
+type: performance

lib/api/branches.rb

@@ -29,12 +29,11 @@ module API
         use :pagination
       end
       get ':id/repository/branches' do
-        branches = ::Kaminari.paginate_array(user_project.repository.branches.sort_by(&:name))
+        repository = user_project.repository
+        branches = ::Kaminari.paginate_array(repository.branches.sort_by(&:name))
+        merged_branch_names = repository.merged_branch_names(branches.map(&:name))
 
-        # n+1: https://gitlab.com/gitlab-org/gitlab-ce/issues/37442
-        Gitlab::GitalyClient.allow_n_plus_1_calls do
-          present paginate(branches), with: Entities::Branch, project: user_project
-        end
+        present paginate(branches), with: Entities::Branch, project: user_project, merged_branch_names: merged_branch_names
       end
 
       resource ':id/repository/branches/:branch', requirements: BRANCH_ENDPOINT_REQUIREMENTS do

lib/api/entities.rb

@@ -244,7 +244,7 @@ module API
       expose :merged do |repo_branch, options|
         # n+1: https://gitlab.com/gitlab-org/gitlab-ce/issues/37442
         Gitlab::GitalyClient.allow_n_plus_1_calls do
-          options[:project].repository.merged_to_root_ref?(repo_branch.name)
+          options[:project].repository.merged_to_root_ref?(repo_branch.name, options[:merged_branch_names])
         end
       end
 

lib/api/v3/branches.rb

@@ -14,9 +14,11 @@ module API
           success ::API::Entities::Branch
         end
         get ":id/repository/branches" do
-          branches = user_project.repository.branches.sort_by(&:name)
+          repository = user_project.repository
+          branches = repository.branches.sort_by(&:name)
+          merged_branch_names = repository.merged_branch_names(branches.map(&:name))
 
-          present branches, with: ::API::Entities::Branch, project: user_project
+          present branches, with: ::API::Entities::Branch, project: user_project, merged_branch_names: merged_branch_names
         end
 
         desc 'Delete a branch'