Adds a `prev_commit_id` and a `next_commit_id`

This should allow for a fronted change to include "Previous" and
"Next" buttons on the commit viewer.

app/serializers/diffs_entity.rb

@@ -3,6 +3,7 @@
 class DiffsEntity < Grape::Entity
   include DiffHelper
   include RequestAwareEntity
+  include Gitlab::Utils::StrongMemoize
 
   expose :real_size
   expose :size
@@ -16,11 +17,19 @@ class DiffsEntity < Grape::Entity
   end
 
   expose :commit do |diffs, options|
+    if options[:commit]
+      neighbors = commits(options[:commit].id) if options[:commit]
+      prev_commit_id = neighbors[:prev_commit_id]
+      next_commit_id = neighbors[:next_commit_id]
+    end
+
     CommitEntity.represent options[:commit], options.merge(
       type: :full,
       commit_url_params: { merge_request_iid: merge_request&.iid },
       pipeline_ref: merge_request&.source_branch,
-      pipeline_project: merge_request&.source_project
+      pipeline_project: merge_request&.source_project,
+      prev_commit_id: prev_commit_id,
+      next_commit_id: next_commit_id
     )
   end
 
@@ -80,4 +89,16 @@ class DiffsEntity < Grape::Entity
   def merge_request
     options[:merge_request]
   end
+
+  def commit_ids
+    strong_memoize(:commit_ids) do
+      [nil] + merge_request.commits.collect(&:id) + [nil]
+    end
+  end
+
+  def commits(id)
+    commit_ids.each_cons(3) do |prev_commit, commit, next_commit|
+      { prev_commit_id: prev_commit, next_commit_id: next_commit } if commit == id
+    end
+  end
 end

lib/api/entities/commit_with_link.rb

@@ -32,6 +32,18 @@ module API
         render('projects/commit/_signature', signature: commit.signature) if commit.has_signature?
       end
 
+      expose :prev_commit_id, if: { type: :full } do |commit|
+        next unless options[:prev_commit_id]
+
+        options[:prev_commit_id]
+      end
+
+      expose :next_commit_id, if: { type: :full } do |commit|
+        next unless options[:next_commit_id]
+
+        options[:next_commit_id]
+      end
+
       expose :pipeline_status_path, if: { type: :full } do |commit, options|
         pipeline_ref = options[:pipeline_ref]
         pipeline_project = options[:pipeline_project] || commit.project

spec/serializers/diffs_entity_spec.rb

@@ -26,5 +26,24 @@ describe DiffsEntity do
         :merge_request_diffs
       )
     end
+
+    context "when a commit_id is passed" do
+      let(:commits) { merge_request.commits }
+      let(:commit) { commits.sample }
+      let(:entity) do
+        described_class.new(merge_request_diffs.first.diffs, request: request, merge_request: merge_request, merge_request_diffs: merge_request_diffs, commit: commit)
+      end
+
+      it 'includes commit references for previous and next' do
+        expect(subject[:commit]).to include(:prev_commit_id, :next_commit_id)
+
+        index = commits.index(commit)
+        prev_commit = commits[index - 1]&.id
+        next_commit = commits[index + 1]&.id
+
+        expect(subject[:commit][:prev_commit_id]).to eq(prev_commit)
+        expect(subject[:commit][:next_commit_id]).to eq(next_commit)
+      end
+    end
   end
 end