Add MR branch back to list view

• Zendesk ticket: https://gitlab.zendesk.com/agent/tickets/10071
• Dev link: None
• GitLab version: 8.0

Prior to 8.0, the merge target branch was visible in the merge request list view. This was removed in 8.0 by 43394f0a. This is critical to the customer's workflow. Please consider adding it back in.