Project.repository should never be nil so you can call repository.exists? or repository.empty?

Also specify separate project factory for project with filled repo

lib/api/projects.rb

@@ -372,7 +372,7 @@ module Gitlab
         ref = params[:ref_name] || user_project.try(:default_branch) || 'master'
 
         commits = user_project.repository.commits(ref, nil, per_page, page * per_page)
-        present CommitDecorator.decorate(commits), with: Entities::RepoCommit
+        present commits, with: Entities::RepoCommit
       end
 
       # Get a project snippets

lib/extracts_path.rb

@@ -85,8 +85,8 @@ module ExtractsPath
   # - @id     - A string representing the joined ref and path
   # - @ref    - A string representing the ref (e.g., the branch, tag, or commit SHA)
   # - @path   - A string representing the filesystem path
-  # - @commit - A CommitDecorator representing the commit from the given ref
-  # - @tree   - A TreeDecorator representing the tree at the given ref/path
+  # - @commit - A Commit representing the commit from the given ref
+  # - @tree   - A Tree representing the tree at the given ref/path
   #
   # If the :id parameter appears to be requesting a specific response format,
   # that will be handled as well.

lib/gitlab/markdown.rb

@@ -187,7 +187,7 @@ module Gitlab
 
     def reference_commit(identifier)
       if @project.valid_repo? && commit = @project.repository.commit(identifier)
-        link_to(identifier, project_commit_url(@project, commit), html_options.merge(title: CommitDecorator.new(commit).link_title, class: "gfm gfm-commit #{html_options[:class]}"))
+        link_to(identifier, project_commit_url(@project, commit), html_options.merge(title: commit.link_title, class: "gfm gfm-commit #{html_options[:class]}"))
       end
     end
   end