Show source and target branches in right sidebar

Description

When I am in a discussion in the middle of the merge request page, sometimes I want to remember what is the current source branch to clone it to my PC, then I have to go to the top of the page to see the following info (and then I loose the focus on the discussion):

Proposal

It would help a lot to show this information all the time, for example in the right sidebar (which has space to fit it)

Feature checklist

Make sure these are completed before closing the issue, with a link to the relevant commit.

• Feature assurance
• Documentation
• Added to features.yml