Improve merge request versions feature

• Add a link to the "Added N commits" note like "Compare with previous version" https://gitlab.com/gitlab-org/gitlab-ce/issues/22773
• Improve compare logic https://gitlab.com/gitlab-org/gitlab-ce/issues/22368, https://gitlab.com/gitlab-org/gitlab-ce/issues/22487
• Consider using interdiff for comparing MR versions https://gitlab.com/gitlab-org/gitlab-ce/issues/23269
• duplicate master in compare https://gitlab.com/gitlab-org/gitlab-ce/issues/22715

Feature itself - https://gitlab.com/gitlab-org/gitlab-ce/issues/13570