Moves some entity code to avoid a fixture error

app/serializers/diffs_entity.rb

@@ -17,20 +17,7 @@ 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,
-      prev_commit_id: prev_commit_id,
-      next_commit_id: next_commit_id
-    )
+    CommitEntity.represent options[:commit], commit_options(options)
   end
 
   expose :context_commits, using: API::Entities::Commit, if: -> (diffs, options) { merge_request&.project&.context_commits_enabled? } do |diffs|
@@ -90,6 +77,8 @@ class DiffsEntity < Grape::Entity
     options[:merge_request]
   end
 
+  private
+
   def commit_ids
     strong_memoize(:commit_ids) do
       [nil] + merge_request.commits.collect(&:id) + [nil]
@@ -101,4 +90,19 @@ class DiffsEntity < Grape::Entity
       { prev_commit_id: prev_commit, next_commit_id: next_commit } if commit == id
     end
   end
+
+  def commit_options(options)
+    neighbors = commits(options[:commit].id)
+    prev_commit_id = neighbors.try(:prev_commit_id)
+    next_commit_id = neighbors.try(:next_commit_id)
+
+    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,
+      prev_commit_id: prev_commit_id,
+      next_commit_id: next_commit_id
+    )
+  end
 end