Fix timeout issues retrieving branches via API

47d4890d changed the order of pagination so that the full list of
branches would be passed to Gitaly to determine which ones had been
merged, but this operation can timeout for large repositories with
many branches. We only need to determine whether the found branches have
been merged, so limit the scan to those.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/55724

changelogs/unreleased/sh-fix-branches-api-timeout.yml

+---
+title: Fix timeout issues retrieving branches via API
+merge_request: 24034
+author:
+type: performance

lib/api/branches.rb

@@ -34,11 +34,11 @@ module API
         repository = user_project.repository
 
         branches = BranchesFinder.new(repository, declared_params(include_missing: false)).execute
-
+        branches = ::Kaminari.paginate_array(branches)
         merged_branch_names = repository.merged_branch_names(branches.map(&:name))
 
         present(
-          paginate(::Kaminari.paginate_array(branches)),
+          paginate(branches),
           with: Entities::Branch,
           current_user: current_user,
           project: user_project,

spec/requests/api/branches_spec.rb

@@ -20,6 +20,12 @@ describe API::Branches do
     let(:route) { "/projects/#{project_id}/repository/branches" }
 
     shared_examples_for 'repository branches' do
+      RSpec::Matchers.define :has_merged_branch_names_count do |expected|
+        match do |actual|
+          actual[:merged_branch_names].count == expected
+        end
+      end
+
       it 'returns the repository branches' do
         get api(route, current_user), params: { per_page: 100 }
 
@@ -30,6 +36,12 @@ describe API::Branches do
         expect(branch_names).to match_array(project.repository.branch_names)
       end
 
+      it 'determines only a limited number of merged branch names' do
+        expect(API::Entities::Branch).to receive(:represent).with(anything, has_merged_branch_names_count(2))
+
+        get api(route, current_user), params: { per_page: 2 }
+      end
+
       context 'when repository is disabled' do
         include_context 'disabled repository'