Status code 404 returned when retrieving non existent branch (issue #2922)

Accessing a repository branch that does not exist returns a 404 error instead
of 200 now. Added a test.

lib/api/projects.rb

@@ -230,6 +230,7 @@ module Gitlab
       #   GET /projects/:id/repository/branches/:branch
       get ":id/repository/branches/:branch" do
         @branch = user_project.repo.heads.find { |item| item.name == params[:branch] }
+        error!("Branch does not exist", 404) if @branch.nil?
         present @branch, with: Entities::RepoObject, project: user_project
       end