Fix API /project/:id/branches not returning correct merge status

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24034 introduced
a regression where only the first 20 branches were used to determine
whether a branch has been merged because the pagination was applied
incorrectly. Requesting the second page of branches via the API would
always have the wrong merge status. We fix this by properly paginating
the branches before requesting their merge status.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/56250

changelogs/unreleased/sh-fix-project-branches-merge-status.yml

+---
+title: Fix API /project/:id/branches not returning correct merge status
+merge_request: 26785
+author:
+type: fixed

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)
+        branches = paginate(::Kaminari.paginate_array(branches))
         merged_branch_names = repository.merged_branch_names(branches.map(&:name))
         present(
-          paginate(branches),
+          branches,
           with: Entities::Branch,
           current_user: current_user,
           project: user_project,

spec/requests/api/branches_spec.rb

@@ -36,10 +36,30 @@ describe API::Branches do
         expect(branch_names).to match_array(project.repository.branch_names)
       end
+      def check_merge_status(json_response)
+        merged, unmerged = json_response.partition { |branch| branch['merged'] }
+        merged_branches = merged.map { |branch| branch['name'] }
+        unmerged_branches = unmerged.map { |branch| branch['name'] }
+        expect(Set.new(merged_branches)).to eq(project.repository.merged_branch_names(merged_branches + unmerged_branches))
+        expect(project.repository.merged_branch_names(unmerged_branches)).to be_empty
+      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))
+        expect(API::Entities::Branch).to receive(:represent).with(anything, has_merged_branch_names_count(1)).and_call_original
         get api(route, current_user), params: { per_page: 2 }
+
+        expect(response).to have_gitlab_http_status(200)
+
+        check_merge_status(json_response)
+      end
+
+      it 'merge status matches reality on paginated input' do
+        get api(route, current_user), params: { per_page: 20, page: 2 }
+
+        expect(response).to have_gitlab_http_status(200)
+
+        check_merge_status(json_response)
       end
       context 'when repository is disabled' do