API: Get refs for a particular commit

What does this MR do?

This merge request adds the functionality to retrieve the refs where a particular commit is included.

Are there points in the code the reviewer needs to double check?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/18014

lib/api/commits.rb

@@ -156,6 +156,31 @@ class Commits < Grape::API
         end
       end
 
+      desc 'Get all reference a commit is pushed to' do
+        detail 'This feature was introduced in GitLab 10.2'
+        success Entities::Ref
+      end
+      params do
+        requires :sha, type: String, desc: 'A commit sha'
+        optional :type, type: String, values: %w(branches tags all), default: 'all', desc: 'Scope'
+      end
+      get ':id/repository/commits/:sha/refs', requirements: API::COMMIT_ENDPOINT_REQUIREMENTS do
+        commit = user_project.commit(params[:sha])
+        not_found!('Commit') unless commit
+
+        case params[:type]
+        when 'branches'
+          refs = user_project.repository.branch_names_contains(commit.id)
+        when 'tags'
+          refs = user_project.repository.tag_names_contains(commit.id)
+        else
+          refs = user_project.repository.branch_names_contains(commit.id)
+          refs << user_project.repository.branch_names_contains(commit.id)
+        end
+
+        present refs, with: Entities::BasicRef
+      end
+
       desc 'Post comment to commit' do
         success Entities::CommitNote
       end